A diminutive swimmer (5)
I believe the answer is:
sprat
This could be a double definition.
'a diminutive' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'swimmer' is the second definition.
(sprat is a kind of fish)
Can you help me to learn more?
(Other definitions for sprat that I've seen before include "'Set a . . . . . to catch a whale'" , "It's set to catch a mackerel" , "Small fish (used to catch mackerel?)" , "Jack --, non-fat eater" , "Small herring-type fish" .)