Restraining expression of disgust is not nothing (6)
I believe the answer is:
nought
'nothing' is the definition.
(synonyms)
'restraining expression of disgust is not' is the wordplay.
'restraining' means one lot of letters goes inside another.
'expression of disgust' becomes 'ugh' (I've seen this before).
'ugh' placed into 'not' is 'NOUGHT'.
(Other definitions for nought that I've seen before include "The digit 0" , "Poetically nothing" , "Nil" , "duck" , "Zilch" .)