Creature whose lair is covered in rubbish (6)
I believe the answer is:
rodent
'creature' is the definition.
(rodent is a kind of creature)
'lair is covered in rubbish' is the wordplay.
'lair' becomes 'den' ('den' can be a synonym of 'lair').
'is covered in' is an insertion indicator.
'rubbish' becomes 'rot' (I've seen this before).
'den' inserted into 'rot' is 'RODENT'.
'whose' acts as a link.
(Other definitions for rodent that I've seen before include "Toothy mammal" , "Animal such as a mouse" , "Mouse maybe" , "A rat, for instance" , "Small gnawing animal" .)