Marine mammals circling for example -- sailors want them (3,4)
I believe the answer is:
sea legs
'sailors want them' is the definition.
'marine mammals circling for example' is the wordplay.
'marine mammals' becomes 'seals' (I've seen this before).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'for example' becomes 'eg' ('e.g.' can be a synonym of 'for example').
'seals' enclosing 'eg' is 'SEA-LEGS'.
(Other definitions for sea legs that I've seen before include "Adaptation to rolling deck" , "Ability to cope with life on board" , "Molten rock" , "Ability to maintain one's balance on a ship" , "Ability in the water" .)