Expression of disgust, ghastly one (7)
I believe the answer is:
grimace
'expression of disgust' is the definition.
(I've seen this before)
'ghastly one' is the wordplay.
'ghastly' 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" , "Ugly facial expression" , "Nasty expression" , "Distortion of the face" , "Twisted facial expression" .)