Fish - without water, without oxygen (4)
I believe the answer is:
dory
'fish' is the definition.
(dory is a kind of fish)
'without water without oxygen' is the wordplay.
'without water' becomes 'dry'.
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'oxygen' becomes 'o' (atomic symbol for oxygen).
'dry' placed around 'o' is 'DORY'.
(Other definitions for dory that I've seen before include "Fish for a small boat" , "Fish of mackerel family" , "Sea-wolf" , "Marine fish; rowing boat" , "another fish wanted" .)