Formerly in concert (4)
I believe the answer is:
'formerly' is the definition.
(I know that formerly can be written as once)
'in concert' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ONCE' is hidden in the letters of 'concert'.
(Other definitions for once that I've seen before include "Some time back" , "On one occasion" , "in bygone era" , "Cone (anag.)" , "Sometime" .)