Formerly in concealment (4)
I believe the answer is:
once
'formerly' is the definition.
('once' can be a synonym of 'formerly')
'in concealment' is the wordplay.
'in' says the answer is hidden in the clue.
'ONCE' is hidden within 'concealment'.
(Other definitions for once that I've seen before include "... upon a time for the storyteller" , "In former times" , "Some time back" , "On a single (or past) occasion" , "At a previous time" .)