Answer in pen, but check (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' means one lot of letters goes inside another.
'pen' becomes 'sty' (sty is a kind of pen).
'a' placed into 'sty' is 'STAY'.
'but' acts as a link.
(Other definitions for stay that I've seen before include "Support, delay" , "control" , "Remain; detain" , "Corset strut" , "hang about" .)