Stream is loud and low (4)
I believe the answer is:
flow
'stream' is the definition.
(I know that stream can be written as flow)
'loud and low' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'and' is a charade indicator (letters next to each other).
'f'+'low'='FLOW'
'is' is the link.
(Other definitions for flow that I've seen before include "Scapa - , Orkney anchorage" , "Uninterrupted stream" , "Run or move like a liquid" , "Trickle" , "Move steadily" .)