Scowl from stern expert (7)
I believe the answer is:
grimace
'scowl' is the definition.
(I've seen this in another clue)
'stern expert' is the wordplay.
'stern' becomes 'grim' (synonyms).
'expert' becomes 'ace' (ace can informally mean very good).
'grim'+'ace'='GRIMACE'
'from' is the link.
(Other definitions for grimace that I've seen before include "unpleasant expression" , "Glower" , "Express wry amusement" , "displeasure" , "Pull a face" .)