Lower margin stops good (very good) service (7)
I believe the answer is:
grimace
'lower' is the definition.
(both can mean to scowl)
'margin stops good very good service' is the wordplay.
'margin' becomes 'rim'.
'stops' indicates putting letters inside.
'good' becomes 'g' (abbreviation).
'very good service' becomes 'ace' (good service in tennis).
'g'+'ace'='gace'
'rim' inserted inside 'gace' is 'GRIMACE'.
(Other definitions for grimace that I've seen before include "Facial expression showing disgust, wry humour etc" , "Nasty expression" , "Contort the face" , "Dour" , "Glower" .)