Ruining rugs, a sweet substance (5)
I believe the answer is:
sugar
'sweet substance' is the definition.
(I've seen this before)
'ruining rugs a' is the wordplay.
'ruining' indicates anagramming the letters.
'rugs'+'a'='rugsa'
'rugsa' is an anagram of 'SUGAR'.
(Other definitions for sugar that I've seen before include "flattery" , "Make more palatable" , "Argus (anag.)" , "White or brown sweetener" , "rotter!" .)