Unscripted talk in theatre stems from garden (7)
I believe the answer is:
rhubarb
This clue could be a double definition.
'unscripted talk in theatre' is the first definition.
(term for unscripted background dialogue)
'stems from garden' is the second definition.
'rhubarb' can be an answer for 'stems' (I have seen 'Edible stems ' mean 'rhubarb' so perhaps 'stems' could also mean 'rhubarb'). I am not certain of the 'from garden' bit.
Can you help me to learn more?
(Other definitions for rhubarb that I've seen before include "Plant with poisonous leaves and edible stems" , "Plant stalks eaten as fruit" , "Plant stems, edible when cooked" , "Nonsense; plant with edible leaf stalks" , "Plant with edible, acidic, leafstalks" .)