Spots knight entering a church (4)
I believe the answer is:
acne
'spots' is the definition.
(skin condition causing spots)
'knight entering a church' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'entering' means one lot of letters goes inside another.
'church' becomes 'ce' (Church of England).
'a'+'ce'='ace'
'n' going inside 'ace' is 'ACNE'.
(Other definitions for acne that I've seen before include "Inflammation of glands in skin" , "Spotty skin condition" , "Skin complaint" , "Skin condition prevalent among teenagers" , "Youthful problem" .)