On reflection, butter cat up to get rodent (6)
I believe the answer is:
marmot
'rodent' is the definition.
(I know that marmot is a type of rodent)
'on reflection butter cat up' is the wordplay.
'on reflection' shows that the letters should be reversed in order.
'butter' becomes 'ram' (a ram is something which 'butts').
'cat' becomes 'tom' (a tom is a male cat).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'ram' back-to-front is 'mar'.
'tom' reversed gives 'mot'.
'mar'+'mot'='MARMOT'
'to get' acts as a link.
(Other definitions for marmot that I've seen before include "Large burrowing rodent" , "Furry burrowing rodent" , "Coarse-furred burrowing rodent" , "wild rodent" , "Small burrowing animal of the squirrel family" .)