Fish not deprived of oxygen in stretch of water (6)
I believe the answer is:
solent
'stretch of water' is the definition.
(I've seen this before)
'fish not deprived of oxygen' is the wordplay.
'fish' becomes 'sole' (sole is a kind of fish**).
'deprived of' is a deletion indicator.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'not' with 'o' taken away is 'nt'.
'sole'+'nt'='SOLENT'
'in' is the link.
(Other definitions for solent that I've seen before include "Waterway between Hampshire and Isle of Wight" , "Water to north of IOW" , "Confine as a prisoner" , "Strait north of Isle of Wight" , "Strait off Portsmouth" .)