Limbless reptiles (6)
I believe the answer is:
snakes
'limbless reptiles' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for snakes that I've seen before include "Reptiles, some venomous" , "Adders, cobras, rattlers etc" , "Serpents" , "Limbless reptiles" , "Eg, cottonmouths, boas" .)