In submarine, maybe, one who’s elected to conserve oxygen is a cold fish (7)
I believe the answer is:
rollmop
'a cold fish' is the definition.
(type of fish fillet)
'in submarine maybe one who's elected to conserve oxygen' is the wordplay.
'in' indicates putting letters inside.
'submarine maybe' becomes 'roll' (submarine is a type of bread roll).
'one who's elected' becomes 'MP' (Member of Parliament).
'to conserve oxygen' becomes 'o' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'roll'+'mp'='rollmp'
'rollmp' enclosing 'o' is 'ROLLMOP'.
'is' acts as a link.
(Other definitions for rollmop that I've seen before include "Curled-up herring fillet" , "Food" , "swimmer that's soused" , "Pickled fillet of raw herring" , "Pickled herring on a stick" .)