Spots first of nuns entering a church (4)
I believe the answer is:
acne
'spots' is the definition.
(I've seen this before)
'first of nuns entering a church' is the wordplay.
'first of' indicates taking the first letters.
'entering' indicates putting letters inside.
'church' becomes 'ce' (abbreviation for Church of England).
The initial letter of 'nuns' is 'n'.
'a'+'ce'='ace'
'n' placed into 'ace' is 'ACNE'.
(Other definitions for acne that I've seen before include "Adolescent spots" , "sort of disease" , "(Inflammation causing) spots" , "Inflammation of glands in skin" , "Adolescent skin disease" .)