Reptile's a misery for a cat (13)
I believe the answer is:
tortoiseshell
'a cat' is the definition.
(tortoiseshell is a kind of cat)
'reptile's a misery' is the wordplay.
'reptile's' becomes 'tortoise's' (tortoise is a kind of reptile).
'a misery' becomes 'hell' (I've seen this in another clue).
'tortoises'+'hell'='TORTOISESHELL'
'for' is the link.
(Other definitions for tortoiseshell that I've seen before include "Domestic cat mottled in brownish-yellow and black" , "Domestic cat - or little shoes (anag)" , "Butterfly - cat" , "The case of the slow-moving" , "Butterfly - yellow-brown mottled material" .)