Snake? Stormy petrel eats one! (7)
I believe the answer is:
reptile
'snake?' is the definition.
(snake is a kind of reptile)
'stormy petrel eats one' is the wordplay.
'stormy' is an anagram indicator.
'eats' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'petrel' with letters rearranged gives 'reptle'.
'reptle' placed around 'i' is 'REPTILE'.
(Other definitions for reptile that I've seen before include "loathsome type" , "Cold-blooded animal like lizard or snake" , "Cold-blooded beast" , "Eg, snake, lizard, etc" , "Lizard, for example" .)