It's sufficient to upset one with an expression of disgust (6)
I believe the answer is:
enough
'it's sufficient' is the definition.
('enough' can be similar in meaning to 'sufficient')
'upset one with an expression of disgust' is the wordplay.
'upset' is a reversal indicator.
'with' says to put letters next to each other.
'an expression of disgust' becomes 'ugh' (I've seen this before).
'one' reversed gives 'eno'.
'eno'+'ugh'='ENOUGH'
'to' is the link.
(Other definitions for enough that I've seen before include "To the required extent" , "The quantity needed, sufficient" , "As many as are required" , "This is going no further!" , "An adequate quantity" .)