Reptile in store -- it desperately gulps oxygen (8)
I believe the answer is:
tortoise
'reptile' is the definition.
(tortoise is a kind of reptile)
'store it desperately gulps oxygen' is the wordplay.
'desperately' indicates an anagram.
'gulps' is an insertion indicator.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'store'+'it'='storeit'
'storeit' anagrammed gives 'trtoise'.
'trtoise' enclosing 'o' is 'TORTOISE'.
'in' acts as a link.
(Other definitions for tortoise that I've seen before include "Shelled creature" , "Reptile with a shell" , "One shelled" , "Lettuce-munching plodder" , "This creature might turn to Tories" .)