Unscripted talk in theatre stems from the garden (7)
I believe the answer is:
rhubarb
This clue is a double definition.
'unscripted talk in theatre' is the first definition.
(term for unscripted background dialogue)
'stems from the garden' is the second definition.
(Other definitions for rhubarb that I've seen before include "rot" , "Plant with edible reddish leafstalks - nonsense" , "extra speech?" , "murmurings in the theatre" , "this stick is to be swallowed" .)