Check answer in pen (4)
I believe the answer is:
stay
'check' is the definition.
(staying is a kind of checking)
'answer in pen' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'in' indicates putting letters inside.
'pen' becomes 'sty' (sty is a kind of pen).
'a' placed within 'sty' is 'STAY'.
(Other definitions for stay that I've seen before include "Sojourn" , "Check" , "Reside" , "Stanchion" , "Period in one place" .)