Mammal's lair covered with rubbish (6)
I believe the answer is:
rodent
'mammal's' is the definition.
(rodent is a kind of mammal)
'lair covered with rubbish' is the wordplay.
'lair' becomes 'den' ('den' can be a synonym of 'lair').
'covered with' is an insertion indicator.
'rubbish' becomes 'rot' (I've seen this before).
'den' put into 'rot' is 'RODENT'.
(Other definitions for rodent that I've seen before include "Small gnawing mammal" , "Nibbler" , "Vermin" , "Eg, capybara" , "Placental mammal having a single pair of incisor, 'gnawing', teeth" .)