Bound, say, to sea change (4)
I believe the answer is:
tide
'sea change' is the definition.
The answer and definition can be both related to natural events as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'bound say' is the wordplay.
'bound' becomes 'tied' ('tie' can be a synonym of 'bind').
'say' indicates a 'sounds like' (homophone) clue.
'tied' sounds like 'TIDE'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for tide that I've seen before include "In the main, movement" , "Periodic water movement" , "Ebb and flow of the sea" , "Movement of seawater" , "One thing that waits for no man" .)