Animal’s lair in decay (6)
I believe the answer is:
rodent
'animal's' is the definition.
(rodent is a kind of animal)
'lair in decay' is the wordplay.
'lair' becomes 'den' (synonyms).
'in' is an insertion indicator.
'decay' becomes 'rot' (rot can mean to decay).
'den' placed into 'rot' is 'RODENT'.
(Other definitions for rodent that I've seen before include "Eg, capybara" , "E.g. hamster" , "Eg, mouse or rat" , "Nibbler" , "Vermin" .)