Jelly portion in tea garden (4)
I believe the answer is:
agar
'jelly' is the definition.
(gelatinous substance)
'portion in tea garden' is the wordplay.
'portion' indicates a hidden word (I've seen 'portion of' mean this).
'AGAR' can be found hidden inside 'in tea garden'.
(Other definitions for agar that I've seen before include "a newspaper rejected" , "setter" , "Medium for growth" , "Food thickening agent obtained from seaweed" , "Jelly got from seaweeds, used in foods" .)