Rodent leading with its tail, and not very big (8)
I believe the answer is:
enormous
'very big' is the definition.
(enormous can mean very large)
I don't understand how the rest of the clue works.
(Other definitions for enormous that I've seen before include "Fat" , "Huge, tremendous" , "Extra-large" , "'Huge, gigantic (8)'" , "Extremely large" .)