Small nail or pin (4)
I believe the answer is:
tack
I believe this is a double definition.
'small nail' is the first definition.
(I've seen this before)
'pin' is the second definition.
(I've seen this before)
(Other definitions for tack that I've seen before include "vulgar" , "(Of a sailing ship) alter course through the wind" , "Nail - direction (eg of boat)" , "Rubbishy things" , "Small nail; sail (boat) against the wind" .)