Mouse perhaps to decay outside lair (6)
I believe the answer is:
rodent
'mouse perhaps' is the definition.
(I know that mouse is a type of rodent)
'decay outside lair' is the wordplay.
'decay' becomes 'rot' (rot can mean to decay).
'outside' means one lot of letters goes inside another.
'lair' becomes 'den' (synonyms).
'rot' going around 'den' is 'RODENT'.
'to' acts as a link.
(Other definitions for rodent that I've seen before include "Vermin" , "Gnawing creature" , "A rat, for instance" , "Small gnawing mammal" , "Placental mammal having a single pair of incisor, 'gnawing', teeth" .)