Animal left under water (4)
I believe the answer is:
seal
'animal' is the definition.
(I have seen 'Marine animal' mean 'seal' so perhaps 'animal' could also mean 'seal')
'left under water' is the wordplay.
'left' becomes 'l' (common abbreviation).
'under' is a charade indicator (letters next to each other).
'water' becomes 'sea' (sea is a kind of water).
'l' after 'sea' is 'SEAL'.
(Other definitions for seal that I've seen before include "Authorise - plug" , "See 7" , "stop leaks" , "Secure; aquatic animal" , "Wax stamp" .)