Formerly concealed content (4)
I believe the answer is:
once
'formerly' is the definition.
(I know that formerly can be written as once)
'concealed content' is the wordplay.
'content' indicates the answer is hidden within the clue (letters contained within the clue).
'ONCE' can be found hidden inside 'concealed'.
(Other definitions for once that I've seen before include "Singly" , "Sometime" , "Just as a single occurrence" , "not repeatedly" , "At some former time" .)