Crested lizard (6)
I believe the answer is:
iguana
'crested lizard' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for iguana that I've seen before include "Tropical tree-lizard" , "Arboreal lizard of S America" , "Lizard Darwin found in the Galapagos" , "Arboreal lizard with a spiny crest" , "Herbivore" .)