Rat's lair in decay (6)
I believe the answer is:
rodent
'rat's' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'lair in decay' is the wordplay.
'lair' becomes 'den' (synonyms).
'in' is an insertion indicator.
'decay' becomes 'rot' (rot can mean to decay).
'den' inserted inside 'rot' is 'RODENT'.
Can you help me to learn more?
(Other definitions for rodent that I've seen before include "Gnawing creature" , "E.g. hamster" , "Toothy mammal" , "Nibbler" , "Creature such as a rat" .)