Bound, we hear, to change in sea level (4)
I believe the answer is:
tide
'change in sea level' is the definition.
The definition and answer can be both related to natural events as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'bound we hear' is the wordplay.
'bound' becomes 'tied' ('tie' can be a synonym of 'bind').
'we hear' indicates a 'sounds like' (homophone) clue.
'tied' sounds like 'TIDE'.
'to' is the link.
Can you help me to learn more?
(Other definitions for tide that I've seen before include "Ocean movement" , "Powerful surge" , "Periodic flow" , "Periodic water movement" , "Periodic rise and fall of the sea" .)