Scowl from unyielding champion (7)
I believe the answer is:
'scowl' is the definition.
(I've seen this in another clue)
'unyielding champion' is the wordplay.
'unyielding' becomes 'grim' (I've seen this in another clue).
'champion' becomes 'ace' (both can mean a person excelling at something).
'from' acts as a link.
(Other definitions for grimace that I've seen before include "Facial expression showing disgust, wry humour etc" , "Dour" , "look disgusted" , "displeasure" , "betray disgust" .)