Sail by an indirect route (4)
I believe the answer is:
tack
This could be a double definition.
'sail' is the first definition.
(I know that tack is a more specific form of the action sail)
'an indirect route' is the second definition.
'tack' can be an answer for 'route' (tack is a kind of route). I'm not certain of the 'an indirect' bit.
'by' is the link.
Can you help me to learn more?
(Other definitions for tack that I've seen before include "Stickiness" , "Rubbishy things" , "Cheap material; nail" , "Stable gear" , "Nail - direction (eg of boat)" .)