Terrible serve results in scowl, perhaps (7)
I believe the answer is:
grimace
'scowl perhaps' is the definition.
'terrible serve' is the wordplay.
'terrible' becomes 'grim'.
'serve' becomes 'ace' (type of tennis serve).
'grim'+'ace'='GRIMACE'
'results in' acts as a link.
(Other definitions for grimace that I've seen before include "Expression of pain or disgust" , "Nasty expression" , "Expression of distaste" , "Distorted facial expression" , "Distortion of the face" .)