Nursery rhyme Jack
I believe the answer is:
sprat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nursery rhyme jack</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)