Be upset – boat's beginning to sink (3)
I believe the answer is:
ebb
'sink' is the definition.
(thesaurus)
'be upset boat's beginning' is the wordplay.
'upset' says the letters should be written backwards.
'beginning' indicates taking the first letters.
The first letter of 'boats' is 'b'.
'be' reversed gives 'eb'.
'eb'+'b'='EBB'
'to' is the link.
(Other definitions for ebb that I've seen before include "Abate" , "Tidal retreat" , "(Of tidewater) recede" , "Withdraw heartless" , "(Sea) recede" .)