Herpetologist's study (7)
I believe the answer is:
reptile
'herpetologist's study' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for reptile that I've seen before include "Snake or crocodile, say" , "Cold-blooded vertebrate" , "Malignant creature" , "Turtle or snake say" , "eg Crocodile" .)