Formerly appearing in concert (4)
I believe the answer is:
once
'formerly' is the definition.
(I know that formerly can be written as once)
'in concert' is the wordplay.
'in' says the answer is hidden in the clue.
'ONCE' is hidden in the letters of 'concert'.
'appearing' acts as a link.
(Other definitions for once that I've seen before include "At some time past" , "... upon a time for the storyteller" , "Long ago" , "Cone (anag.)" , "not repeatedly" .)