Snake study in vast country not starting (8)
I believe the answer is:
anaconda
'snake' is the definition.
(anaconda is a kind of snake)
'study in vast country not starting' is the wordplay.
'study' becomes 'con' (con can mean to study).
'in' means one lot of letters goes inside another.
'vast country' becomes 'canada' (Canada is a large country).
'not starting' means to remove the first letter.
'canada' with its first letter removed is 'anada'.
'con' going inside 'anada' is 'ANACONDA'.
(Other definitions for anaconda that I've seen before include "1 of 21 [SNAKES]" , "Boa constrictor" , "Water-boa" , "Water boa" , "The largest snake" .)