Loud and deep is the current (4)
I believe the answer is:
flow
'the current' is the definition.
('flow' can be a synonym of 'current')
'loud and deep' is the wordplay.
'loud' becomes 'f' (A musical term).
'and' is a charade indicator (letters next to each other).
'deep' becomes 'low' ('low' can be similar in meaning to 'deep').
'f'+'low'='FLOW'
'is' acts as a link.
(Other definitions for flow that I've seen before include "Move along as a stream" , "Wolf (anag.)" , "Fluid motion" , "Run or move like a liquid" , "Current" .)