Ruffle feathers initially next to small brook (5)
I believe the answer is:
frill
'ruffle' is the definition.
(I've seen this before)
'feathers initially next to small brook' is the wordplay.
'initially' indicates taking the first letters.
'next to' says to put letters next to each other.
'small brook' becomes 'rill' (I've seen this before).
The initial letter of 'feathers' is 'f'.
'f'+'rill'='FRILL'
(Other definitions for frill that I've seen before include "Superfluous ornament; ruffle" , "Gathered material" , "Tuck" , "Ruche; ruff" , "Trim" .)