Gnawing mammal in rubbish outside dive (6)
I believe the answer is:
rodent
'gnawing mammal' is the definition.
(I've seen this before)
'rubbish outside dive' is the wordplay.
'rubbish' becomes 'rot' (both can mean nonsense).
'outside' indicates putting letters inside.
'dive' becomes 'den' (I've seen this in other clues).
'rot' enclosing 'den' is 'RODENT'.
'in' acts as a link.
(Other definitions for rodent that I've seen before include "Creature such as a rat" , "Small gnawing mammal" , "Mouse or squirrel, say" , "Gnawing creature" , "Eg, capybara" .)