Rodent's to harm retreating cat (6)
I believe the answer is:
marmot
'rodent's' is the definition.
(marmot is a kind of rodent)
'harm retreating cat' is the wordplay.
'harm' becomes 'mar' (I've seen this before).
'retreating' is a reversal indicator.
'cat' becomes 'tom' (a tom is a male cat).
'tom' back-to-front is 'mot'.
'mar'+'mot'='MARMOT'
'to' is the link.
(Other definitions for marmot that I've seen before include "Small burrowing animal of the squirrel family" , "Coarse-furred burrowing rodent" , "Large burrowing rodent" , "Furry burrowing rodent" , "wild rodent" .)