Diminutive swimmers baiting greater ones (6)
I believe the answer is:
sprats
'ones' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for sprats that I've seen before include "Small fishes used to catch larger ones" , "Small fish like sardines" , "Couple fussy about meat" , "Small fish used to catch larger ones" , "Small fish, set to catch whales" .)