Pudding thickener (7)
I believe the answer is:
tapioca
'pudding thickener' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tapioca that I've seen before include "Foodstuff used as a thickener" , "course" , "Pudding made from an extract of cassava" , "Ciao, Pat, have some strange pudding" , "Sago pudding" .)