Small fish used to catch larger ones (6)
I believe the answer is:
sprats
'small fish used to catch larger ones' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sprats that I've seen before include "Small marine fish of the herring family" , "Use them to catch mackerel" , "'Fish, like small herrings (6)'" , "Small fish used to catch larger ones" , "Couple fussy about meat" .)