Rodent with reason to keep to itself (9)
I believe the answer is:
groundhog
'rodent' is the definition.
(I know that groundhog is a type of marmot)
'with reason to keep to itself' is the wordplay.
'with' means one lot of letters go next to another.
'reason' becomes 'ground' (synonyms).
'to keep to itself' becomes 'hog' (as in hogging something).
'ground'+'hog' is 'GROUNDHOG'.
(Other definitions for groundhog that I've seen before include "rodent" , "Burrowing animal" , "US native" , "Woodchuck" .)