Keep quiet, flexible bushes (9)
I believe the answer is:
shrubbery
'bushes' is the definition.
(I've seen this before)
'keep quiet flexible' is the wordplay.
'keep quiet' becomes 'sh' (as in shushing someone).
'flexible' becomes 'rubbery' (I've seen this before).
'sh'+'rubbery'='SHRUBBERY'
(Other definitions for shrubbery that I've seen before include "Garden site of small woody plants" , "Bushy garden area" , "collection of plants" , "That might go in bed" , "Rosemary's place" .)