A hook at the back splitting top sail (9)
I believe the answer is:
spinnaker
'sail' is the definition.
(spinnaker is a kind of sail)
'a hook at the back splitting top' is the wordplay.
'at the back' suggests the final letters.
'splitting' indicates putting letters inside.
'top' becomes 'spinner' (as in a playing top**).
The last letter of 'hook' is 'k'.
'a'+'k'='ak'
'ak' inserted within 'spinner' is 'SPINNAKER'.
(Other definitions for spinnaker that I've seen before include "one using the breeze" , "Large triangular sail at the front of a yacht" , "Lightweight sail" , "Large three-cornered sail" , "Type of sail" .)