Foursome in concert on a single occasion (4)
I believe the answer is:
once
'on a single occasion' is the definition.
(I've seen this before)
'foursome in concert' is the wordplay.
'foursome in' indicates the answer is hidden within the clue (four letters hidden in the clue).
'ONCE' can be found hidden inside 'concert'.
(Other definitions for once that I've seen before include "In former times" , "At a previous time" , "Cone (anag.)" , "in bygone era" , "Formerly; as soon as" .)