Scowls when dirt's covering air conditioner (8)
I believe the answer is:
grimaces
'scowls' is the definition.
(I've seen this in another clue)
'dirt's covering air conditioner' is the wordplay.
'dirt's' becomes 'grime's' (grime can mean dirt or mud).
'covering' is an insertion indicator.
'air conditioner' becomes 'ac' (this could be a standard abbreviation which I don't know about).
'grimes' going around 'ac' is 'GRIMACES'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for grimaces that I've seen before include "Facial distortions of distaste" , "threatening features" , "Makes wry expression" , "Contortion of face, perhaps expressing pain" , "Twisted expressions" .)