Smirk from unpleasant expert (7)
I believe the answer is:
grimace
'smirk' is the definition.
(thesaurus)
'unpleasant expert' is the wordplay.
'unpleasant' becomes 'grim' ('grim' can be similar in meaning to 'unpleasant').
'expert' becomes 'ace' (ace is a kind of expert).
'grim'+'ace'='GRIMACE'
'from' is the link.
(Other definitions for grimace that I've seen before include "Distortion of the face" , "Expression of distaste" , "betray disgust" , "Dour" , "Twisted facial expression" .)