A portion of rhubarb jam (5)
I believe the answer is:
stick
'jam' is the definition.
(jam can mean to get stuck)
'a portion of rhubarb' is the wordplay.
I cannot really see how this works, but
'portion' could be 'ti' (ti is a kind of portion) and 'ti' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stick that I've seen before include "Staff - criticism" , "Gear lever; adhere" , "School punishment once" , "Cane; criticism" , "heavy criticism" .)