Booker? With prize ultimately secured, trim one's sails (4)
I believe the answer is:
reef
'trim one's sails' is the definition.
(sailing term)
'booker? with prize ultimately' is the wordplay.
'booker?' becomes 'ref' (a football referee can book players).
'with' means one lot of letters goes inside another.
'ultimately' says to take the final letters.
The last letter of 'prize' is 'e'.
'ref' placed around 'e' is 'REEF'.
'secured' acts as a link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for reef that I've seen before include "The sort of knot" , "Shorten sail" , "Undersea ridge; reduce sail area" , "Submerged barrier" , "Ridge of coral near the sea surface" .)