Balance for one second on large animal (3,4)
I believe the answer is:
sea legs
'balance' is the definition.
(balance on a ship)
'for one second on large animal' is the wordplay.
'for one' becomes 'eg' (both can mean 'for example').
'second' becomes 's' (common abbreviation).
'on' says to put letters next to each other.
'large animal' becomes 'seal' (aquatic animal).
'eg'+'s'='egs'
'egs' after 'seal' is 'SEA-LEGS'.
(Other definitions for sea legs that I've seen before include "Stability on board ship" , "Ability to maintain one's balance on a ship" , "Balance on board" , "Ability to cope with life on board" , "not to feel sick on board?" .)