Small marine fish (3,5)
(Other definitions for sea horse that I've seen before include "Upright swimmer" , "Small fish with equine-like head." , "Small marine creature of equine nomenclature" , "Small fish swimming upright" , "Marine equine mammal" .)
(Other definitions for sardine that I've seen before include "Young pilchard" , "A diner's small fish" , "Rinsed a (anag) - fish" , "Swimmer" , "Fish (often tinned)" .)
'small marine fish' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sprat that I've seen before include "Young herring" , "chap on a fat-free diet" , "chap giving richer course material to the Dutch" , "Parts (anag.)" , "Small herring-type fish" .)