Quietly supported by act protecting western water plant (8)
I believe the answer is:
pondweed
'water plant' is the definition.
(pondweed is a kind of water plant)
'quietly supported by act protecting western' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'supported by' becomes 'on'.
'act' becomes 'deed' (deed is a kind of act).
'protecting' means one lot of letters goes inside another.
'western' becomes 'w'.
'deed' placed around 'w' is 'dweed'.
'p'+'on'+'dweed'='PONDWEED'
(Other definitions for pondweed that I've seen before include "water plant" , "Aquatic plant" .)