Rodent damage returning to motorway (6)
I believe the answer is:
marmot
'rodent' is the definition.
(marmot is a kind of rodent)
'damage returning to motorway' is the wordplay.
'damage' becomes 'mar' (marring is a kind of damaging).
'returning' says the letters should be written backwards.
'motorway' becomes 'm'.
'to'+'m'='tom'
'tom' written backwards gives 'mot'.
'mar'+'mot'='MARMOT'
(Other definitions for marmot that I've seen before include "Small burrowing animal of the squirrel family" , "Furry burrowing rodent" , "Stout, burrowing rodent" , "wild rodent" , "Large burrowing rodent" .)