From the evidence in your source, the first use was in the Blackmoor Campaign.
Based on Rob Kuntz' notes in the source you cited, Oerth Journal, 6, p. 52, its origin and first use before 1975 can only be attested to by veterans of Arneson's campaign: Dave Arneson was running his Blackmoor campaign as far back as 1972, or even 1971. Per Kuntz' comment, Dave Arneson is the clone spell's inventor.
From the Oerrth journal entry, Rob Kuntz:
The adventure took place in 1976 at TSR's Dungeon Hobby Shop. Gary,
myself and Dave Arneson were the only participants. Dave's campaign
had been running for some time previous to this and many adventures
into that fabled city had taken place before ours. Dave had in fact
published a Blackmoor piece with an accompanying map in the long
defunct newsletter for the Castle and Crusade Society, "Domesday
Book," issue #13, 1972 (Editors/Founders: Gygax/Kuntz).
{snip} The cross netted over 100,000 gold; the scrolls were all clone spells
(Dave's invention); and the candles were "Nobelite" (i.e, dynamite).
You can state with some confidence that player(s) in the Blackmoor dungeon/campaign in the Twin Cities area (well before this session in 1976) would have been involved in the play, and play test, of Dave's invention, since clone was published in 1975 in the Greyhawk book (for which Arneson did not get "name" publishing credit) but after the original spell list in the original D&D, Men and Magic (TSR, 1974).
"Facts about Black Moor" for Domesday Book #13, which brought his
innovations to the attention of the rest of the Castle & Crusade
Society. That fall, Arneson demonstrated the game for Gygax, and work
on Dungeons & Dragons commenced. As rule development proceeded, the
Blackmoor campaign continued, and began coordinating with a parallel
campaign known as Greyhawk run out of Lake Geneva by Gygax and his
circle.[4] After the publication of Dungeons & Dragons, the Blackmoor
campaign continued, but as a number of key participants (including
Arneson) left Minneapolis to work in Lake Geneva, play of the campaign
grew more sporadic. (source)
- 8th level spells were not in the published game as of 1974; whether Dave created the spell before or after original publishing of D&D can only be known by those who played in his Blackmoor campaign.
- The clone spell's import into published D&D via Greyhawk Supplement (1975) suggests X amount of play or play test before that date and after the creation of the Blackmoor castle/campaign around 1972, before 1975, during which time Dave came up with that spell in his campaign.
The first in game use of the clone spell is unknowable. Its first publication as an 8th level spell was in Greyhawk, OD&D Supplement 1, spring 1975, TSR.
Greyhawk was already in process at the time of TSR co-founder Don
Kaye's death in January 1975, and was published a few months later in
the spring of 1975.
The Wikipedia entry squares with my experience, having begun D&D in summer/fall of 1975; Greyhawk was already available (clone was/is on p. 26). We used the clone spell during play in 1975. (It's a good thing we did, or my original character would never have survived to 9th level. Thanks to clone, we made that NPC wizard rich ...) Paying a wizard for a clone spell out of our treasure was one of the few ways we had of recovering dead party members, who came back as their "previous self" (no XP for the fatal raid) due to how that 8th level spell worked.