High quiet stream (6)
I believe the answer is:
shrill
'high' is the definition.
(similar in meaning)
'quiet stream' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'stream' becomes 'rill' (rill is a kind of stream).
'sh'+'rill'='SHRILL'
(Other definitions for shrill that I've seen before include "Strident" , "Of voice, highl-pitched and loud" , "(Of a cry) piercing" , "Loud and high-pitched" , "Keen" .)