Appeared in concert on a single occasion (4)
I believe the answer is:
once
'on a single occasion' is the definition.
(I've seen this in another clue)
'appeared in concert' is the wordplay.
'appeared in' says the answer is hidden in the clue (I've seen 'appearing in' mean this).
'ONCE' is hidden in the letters of 'concert'.
(Other definitions for once that I've seen before include "in days of yore?" , "Some time back" , "in bygone era" , "One-time - one time" , "Formerly - not repeated" .)