Formerly in concert (4)
I believe the answer is:
once
'formerly' is the definition.
(synonyms)
'in concert' is the wordplay.
'in' indicates the answer is hidden within 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" .)