Belle won't want this expert covering north (4)
I believe the answer is:
acne
'belle won't want this' is the definition.
The definition suggests a singular noun which matches the answer.
'expert covering north' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'covering' means one lot of letters goes inside another.
'north' becomes 'N' (abbreviation).
'ace' going around 'n' is 'ACNE'.
Can you help me to learn more?
(Other definitions for acne that I've seen before include "Teenage spots" , "Facial spots" , "Spotty skin condition" , "Pimply ailment of adolescnce" , "Face problems" .)