A little fish, or sea animal (5)
I believe the answer is:
horse
'animal' is the definition.
(type of animal)
'a little fish or sea' is the wordplay.
'a little' says the answer is hidden in the clue (a little bit of the clue).
'HORSE' is hidden in the letters of 'fish or sea'.
(Other definitions for horse that I've seen before include "Eg, mare" , "charger?" , "drug?" , "Mount" , "Animal; cavalry" .)