Boat that has fortified wine, heading 90 degrees off-course (6)
I believe the answer is:
wherry
'boat' is the definition.
(wherry is a kind of boat)
I cannot understand how the rest of the clue works.
(Other definitions for wherry that I've seen before include "Harbour rowing boat" , "Shallow boat" , "Light rowing vessel for passengers" , "Barge; small rowing-boat" , "Light rowing boat" .)