Stern expert with a distorted facial expression (7)
I believe the answer is:
grimace
'a distorted facial expression' is the definition.
(I've seen this before)
'stern expert' is the wordplay.
'stern' becomes 'grim' (synonyms).
'expert' becomes 'ace' (ace can informally mean very good).
'grim'+'ace'='GRIMACE'
'with' acts as a link.
(Other definitions for grimace that I've seen before include "Expression of pain or disgust" , "Express wry amusement" , "Contortion of face expressing pain" , "displeasure" , "Expression of distaste" .)