S-sailor upset to see these leaving ship (4)
I believe the answer is:
rats
'see these leaving ship' is the definition.
's-sailor upset' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'upset' shows that the letters should be reversed in order.
's'+'tar'='star'
'star' back-to-front is 'RATS'.
'to' is the link.
(Other definitions for rats that I've seen before include "Snoopy's imprecation - vermin" , "(Hunts) rodents - expression of irritation" , "Mild oath - sewer residents" , "Annoyed expression" , "were piped away" .)