Ease when floating, for example, among marine mammals (3,4)
I believe the answer is:
sea legs
'ease when floating' is the definition.
I can't judge whether this defines the answer.
'example among marine mammals' is the wordplay.
'example' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'marine mammals' becomes 'seals' (aquatic animals).
'eg' placed inside 'seals' is 'SEA-LEGS'.
'for' is the link.
Can you help me to learn more?
(Other definitions for sea legs that I've seen before include "musical" , "Ability to cope with life on board" , "not to feel sick on board?" , "Adaptation to rolling deck" , "What a good sailor has" .)