Loud stream for decoration (5)
I believe the answer is:
frill
'for decoration' is the definition.
(frill is a kind of decoration)
'loud stream' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'stream' becomes 'rill' (rill is a kind of stream).
'f'+'rill'='FRILL'
(Other definitions for frill that I've seen before include "A decorative gathering of fabric" , "Ruff, adornment" , "Gathered strip as a trimming" , "Unnecessary embellishment" , "Superfluous ornament; ruffle" .)