Arboreal rodent (8)
I believe the answer is:
squirrel
'arboreal rodent' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for squirrel that I've seen before include "Grey or (rarely) red woodland creature" , "Tree-climbing rodent with bushy tail, red or grey" , "Gnawing animal" , "Red, grey, animal" , "Nut-gathering rodent" .)