Cutting rent off with a song (9)
I believe the answer is:
trenchant
'cutting' is the definition.
(I've seen this before)
'rent off with a song' is the wordplay.
'off' is an anagram indicator.
'with' is a charade indicator (letters next to each other).
'a song' becomes 'chant' (song sung by a crowd).
'rent' anagrammed gives 'tren'.
'tren'+'chant'='TRENCHANT'
(Other definitions for trenchant that I've seen before include "Forthright, vigorous" , "Vigorous and effective" , "Sharp" , "Keen" , "blunt" .)