Spots a knight in church (4)
I believe the answer is:
acne
'spots' is the definition.
(I've seen this before)
'a knight in church' is the wordplay.
'knight' becomes 'n' (chess).
'in' indicates putting letters inside.
'church' becomes 'ce' (abbreviation for Church of England).
'n' going into 'ce' is 'cne'.
'a'+'cne'='ACNE'
(Other definitions for acne that I've seen before include "Spotty skin condition" , "Spotty coverage" , "Teenage spots" , "Adolescent skin disease" , "Spotty complaint" .)