Hippocampus (3,5)
I believe the answer is:
sea horse
'hippocampus' is the definition.
(I know that hippocampus can be written as sea-horse)
This is all the clue.
(Other definitions for sea horse that I've seen before include "Marine fish" , "Small marine creature of equine nomenclature" , "Something fishy here" , "Fish, Hippocampus" , "Marine equine mammal" .)