The animal has a lair inside? Nonsense! (6)
I believe the answer is:
'the animal' is the definition.
(rodent is a kind of animal)
'a lair inside? nonsense' is the wordplay.
'a lair' becomes 'den' ('den' can be a synonym of 'lair').
'inside?' means one lot of letters goes inside another.
'nonsense' becomes 'rot' (rot can mean nonsense or rubbish).
'den' going into 'rot' is 'RODENT'.
'has' acts as a link.
(Other definitions for rodent that I've seen before include "Mouse or squirrel, say" , "Toothy mammal" , "Gnawing creature" , "Rat perhaps" , "Small gnawing mammal" .)