Legless reptiles (6)
I believe the answer is:
snakes
'legless reptiles' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for snakes that I've seen before include "Adders, cobras, rattlers etc" , "[CREES]" , "Scaly reptiles" , "Legless reptiles" , "Limbless reptiles" .)