Keen husband dipped into small brook (6)
I believe the answer is:
shrill
'keen' is the definition.
(I've seen this before)
'husband dipped into small brook' is the wordplay.
'husband' becomes 'h'.
'dipped into' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'brook' becomes 'rill' (I've seen this before).
's'+'rill'='srill'
'h' put inside 'srill' is 'SHRILL'.
(Other definitions for shrill that I've seen before include "High and piercing" , "Strident" , "Of voice, highl-pitched and loud" , "Of sound, piercing and high-pitched" , "High-pitched and sharp" .)