Fish without water absorbs oxygen (4)
I believe the answer is:
dory
'fish' is the definition.
(dory is a kind of fish)
'without water absorbs oxygen' is the wordplay.
'without water' becomes 'dry'.
'absorbs' means one lot of letters goes inside another.
'oxygen' becomes 'O' (O is the chemical symbol for oxygen).
'dry' placed around 'o' is 'DORY'.
(Other definitions for dory that I've seen before include "Marine fish; rowing boat" , "Fish and flat-bottomed boat" , "Golden-yellow fish" , "Fish for a small boat" , "Fish of mackerel family" .)