Boat's tack by north star (7)
I believe the answer is:
pinnace
'boat's' is the definition.
(historical type of small boat)
'tack by north star' is the wordplay.
'tack' becomes 'pin' (I've seen this before).
'by' means one lot of letters go next to another.
'north' becomes 'n' ('N' can be a synonym of 'north').
'star' becomes 'ace' ('ace' can be a synonym of 'star').
'pin'+'n'+'ace'='PINNACE'
(Other definitions for pinnace that I've seen before include "Vessel" , "Ship's boat - nice nap (anag)" , "Small boat" , "tender" .)