Edible stems (7)
I believe the answer is:
'edible stems' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rhubarb that I've seen before include "An edible red stalk" , "Leaf-stalks eaten with crumble" , "Stalks edible when cooked" , "mutterings in the theatre" , "Reddish leafstalks cooked and eaten as fruit" .)