Forbidding one expression of disgust (7)
I believe the answer is:
grimace
'expression of disgust' is the definition.
(I've seen this before)
'forbidding one' is the wordplay.
'forbidding' becomes 'grim' (I've seen this before).
'one' becomes 'ace' ('ace' can be a synonym of 'one').
'grim'+'ace'='GRIMACE'
(Other definitions for grimace that I've seen before include "Expression of pain or disgust" , "Distorted facial expression" , "Facial contortion expressive of pain or disgust" , "displeasure" , "(Make) a wry face" .)