Scowl from serious expert (7)
I believe the answer is:
grimace
'scowl' is the definition.
(I've seen this in another clue)
'serious expert' is the wordplay.
'serious' becomes 'grim'.
'expert' becomes 'ace' (ace can informally mean very good).
'grim'+'ace'='GRIMACE'
'from' acts as a link.
(Other definitions for grimace that I've seen before include "Distortion of the face" , "Contort the face" , "Glower" , "Pull a face" , "displeasure" .)