Change course of small nail (4)
I believe the answer is:
tack
This clue is a double definition.
'change course' is the first definition.
(I've seen this in another clue)
'small nail' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for tack that I've seen before include "Keep changing course" , "Short, flat-headed nail" , "Stickiness" , "Rubbishy things" , "vulgar" .)