Nursery rhyme Jack
I believe the answer is:
sprat
'nursery rhyme jack' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sprat that I've seen before include "Small fish (used to catch mackerel?)" , "Mackerel bait?" , "one on a fat-free diet" , "little fish" , "chap on a fat-free diet" .)