A kid going back for jelly (4)
I believe the answer is:
agar
'jelly' is the definition.
(gelatinous substance)
'a kid going back' is the wordplay.
'kid' becomes 'rag' (I've seen this in other clues).
'going back' shows that the letters should be reversed in order.
'rag' written backwards gives 'gar'.
'a'+'gar'='AGAR'
'for' is the link.
(Other definitions for agar that I've seen before include "that aids culture" , "setter" , "a newspaper rejected" , "Jelly got from seaweeds, used in foods" , "Food thickener (derived from seaweed)" .)