Cord attaching boat's stern to port, say (5)
I believe the answer is:
twine
'cord' is the definition.
(twine is a kind of cord)
'attaching boat's stern to port say' is the wordplay.
I cannot quite understand how this works, but
'port say' could be 'wine' (port is a kind of wine) and 'wine' is found in the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for twine that I've seen before include "Tough string" , "Strong thread or string" , "Interweave" , "Thin cord" , "String, cord" .)