Bound to be heard as ebb and flow (4)
I believe the answer is:
tide
'ebb and flow' is the definition.
(I've seen this before)
'bound to be heard' is the wordplay.
'bound' becomes 'tied' (similar in meaning).
'to be heard' shows a homophone (sound like).
'tied' is a homophone of 'TIDE'.
'as' is the link.
(Other definitions for tide that I've seen before include "''There is a ... in the affairs of men'' (Julius Caesar)" , "Sea's movement" , "Regular movement" , "Ebb and flow of water" , "In the main, movement" .)