Forbidding expert to sneer (7)
I believe the answer is:
grimace
'sneer' is the definition.
(I know that sneer is a more specific form of the action smile)
'forbidding expert' is the wordplay.
'forbidding' becomes 'grim' (I've seen this before).
'expert' becomes 'ace' (ace can informally mean very good).
'grim'+'ace'='GRIMACE'
'to' is the link.
(Other definitions for grimace that I've seen before include "Smirk" , "Dour" , "Contortion of face expressing pain" , "Distortion of the face" , "Pull a face" .)