Formerly 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 within 'concert'.
(Other definitions for once that I've seen before include "Some time back" , "On one occasion" , "in bygone era" , "Cone (anag.)" , "Sometime" .)