Run loud and deep (4)
I believe the answer is:
flow
'run' is the definition.
(synonyms)
'loud and deep' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'and' is a charade indicator (letters next to each other).
'deep' becomes 'low' (similar in meaning).
'f'+'low'='FLOW'
(Other definitions for flow that I've seen before include "Run smoothly" , "Wolf (anag.)" , "Run or move like a liquid" , "(Forward) movement of liquid" , "Fluid motion" .)