Advanced kid rejected jelly (4)
I believe the answer is:
agar
'jelly' is the definition.
(gelatinous substance)
'advanced kid rejected' is the wordplay.
'advanced' becomes 'a' (abbreviation).
'kid' becomes 'rag' (both can mean to tease).
'rejected' shows that the letters should be reversed in order.
'rag' in reverse letter order is 'gar'.
'a'+'gar'='AGAR'
(Other definitions for agar that I've seen before include "that aids culture" , "Food thickener (derived from seaweed)" , "Jelly medium" , "Food thickening agent obtained from seaweed" , "Seaweed derivative" .)