Boat to swing off course to port (4)
I believe the answer is:
yawl
'boat' is the definition.
(yawl is a kind of boat)
'swing off course to port' is the wordplay.
'swing off course' becomes 'yaw' (I can't justify this - if you can you should believe this answer much more).
'to port' becomes 'L' (abbreviation for L - 'port' is 'left' on a boat).
'yaw'+'l'='YAWL'
'to' is the link.
Can you help me to learn more?
(Other definitions for yawl that I've seen before include "Sailing vessel with two masts" , "Sailing-ship" , "Two-masted sailing vessel" , "Ships small boat" , "boat? The reverse" .)