Rat to go off with puss catching nothing (8)
I believe the answer is:
turncoat
'rat' is the definition.
(thesaurus)
'go off with puss catching nothing' is the wordplay.
'go off' becomes 'turn' (I've seen this before**).
'with' is a charade indicator (letters next to each other).
'puss' becomes 'cat' (I've seen this in other clues).
'catching' is an insertion indicator.
'nothing' becomes 'o' (looks like zero - 0).
'cat' going around 'o' is 'coat'.
'turn'+'coat'='TURNCOAT'
'to' acts as a link.
(Other definitions for turncoat that I've seen before include "'Renegade, traitor (8)'" , "Actor nut may be a traitor" , "Deserter to opposition" , "Traitor with inverted garment" , "Traitor with reversed garment" .)