It's bound, we hear, to be current (4)
I believe the answer is:
tide
'be current' is the definition.
(sea currents)
'it's bound we hear' is the wordplay.
'it's bound' becomes 'tied' ('tie' can be a synonym of 'bind').
'we hear' shows a homophone (sound like).
'tied' sounds like 'TIDE'.
'to' is the link.
(Other definitions for tide that I've seen before include "Periodic flow" , "Something going in and out" , "Current, ebb" , "What might carry" , "Movement of seawater" .)