Rodent, cat and farm animal, climbing (6)
I believe the answer is:
marmot
'rodent' is the definition.
(marmot is a kind of rodent)
'cat and farm animal climbing' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'and' says to put letters next to each other.
'farm animal' becomes 'ram' (I've seen this before).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'tom'+'ram'='tomram'
'tomram' back-to-front is 'MARMOT'.
(Other definitions for marmot that I've seen before include "Small burrowing animal of the squirrel family" , "wild rodent" , "Coarse-furred burrowing rodent" , "Creature" , "Large burrowing rodent" .)