Set out from port (4)
I believe the answer is:
left
This clue is a double definition.
'set out' is the first definition.
(I've seen this before)
'port' is the second definition.
('left' can be a synonym of 'port')
'from' is the link.
(Other definitions for left that I've seen before include "Socialist direction" , "The side of socialism" , "Sailed from" , "walk-out?" , "The sinister side" .)