Some insects are ruinous to rugs (7)
I believe the answer is:
earwigs
'some insects' is the definition.
(earwig is a kind of insect)
'are ruinous to rugs' is the wordplay.
'ruinous' indicates an anagram.
'to rugs' becomes 'wigs' (rug is a slang term for a wig).
'are' anagrammed gives 'ear'.
'ear'+'wigs'='EARWIGS'
(Other definitions for earwigs that I've seen before include "listens in furtively" , "Elongated insects" , "Eavesdrops on the insects" , "Bugs" , "Insect with pincers" .)