Spot area knight has captured (4)
I believe the answer is:
daub
'spot' is the definition.
(I know that spot can be written as daub)
'area knight has captured' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'knight' becomes 'dub' (to dub can mean to knight someone).
'has captured' is an insertion indicator.
'a' placed within 'dub' is 'DAUB'.
(Other definitions for daub that I've seen before include "Crude painting" , "Carelessly spread (paint)" , "Smear; bad painting" , "Plaster; paint crudely" , "coat casually" .)