Spots knight lying in a church (4)
I believe the answer is:
acne
'spots' is the definition.
(I've seen this before)
'knight lying in a church' is the wordplay.
'knight' becomes 'n' (chess).
'lying in' is an insertion indicator.
'church' becomes 'ce' (abbreviation for Church of England).
'a'+'ce'='ace'
'n' put within 'ace' is 'ACNE'.
(Other definitions for acne that I've seen before include "Skin affliction" , "Adolescent spots" , "Problem to face in" , "Skin disorder" , "Pimply skin condition of adolescence" .)