Serious expert's dissatisfied look (7)
I believe the answer is:
grimace
'dissatisfied look' is the definition.
(grimace can mean a look of dissatisfaction)
'serious expert's' is the wordplay.
'serious' becomes 'grim'.
'expert' becomes 'ace' (ace can informally mean very good).
'grim'+'ace'='GRIMACE'
(Other definitions for grimace that I've seen before include "Frown" , "Wry look" , "Pained expression" , "Facial contortion expressive of pain or disgust" , "Distorted facial expression" .)