Regularly tack in top sail (9)
I believe the answer is:
spinnaker
'sail' is the definition.
(spinnaker is a kind of sail)
'regularly tack in top' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'in' indicates putting letters inside.
'top' becomes 'spinner' (as in a playing top).
The alternate letters of 'tack' are 'ak'.
'ak' inserted into 'spinner' is 'SPINNAKER'.
(Other definitions for spinnaker that I've seen before include "Large racing sail" , "Large three-cornered sail" , "Large triangular sail at the front of a yacht" , "Type of sail" , "Lightweight sail" .)