Unrefined rot mostly: that's disgusting! (5)
I believe the answer is:
rough
'unrefined' is the definition.
(similar in meaning)
'rot mostly that's disgusting' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'that's disgusting' becomes 'ugh' (expression of disgust).
'rot' with its last letter taken away is 'ro'.
'ro'+'ugh'='ROUGH'
(Other definitions for rough that I've seen before include "Having uneven or irregular surface, say part of golf course" , "Not level or smooth, like part of golf course" , "that driver tries to avoid" , "Rasping" , "Nasty" .)