Loud stream is an unnecessary extra (5)
I believe the answer is:
frill
'an unnecessary extra' is the definition.
(I've seen this in another clue)
'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'
'is' is the link.
(Other definitions for frill that I've seen before include "Trim" , "Maybe superfluous piece of adornment" , "Ruche; ruff" , "Tuck" , "Superfluous ornament; ruffle" .)