Sailing vessel, say, with 100 on (6)
I believe the answer is:
cutter
'sailing vessel' is the definition.
(I know that cutter is a type of sailing vessel)
'say with 100 on' is the wordplay.
'say' becomes 'utter' (to utter something is to speak or say it).
'with c00' becomes 'c' (Roman numeral. I am not sure about the 'with' bit.).
'on' says to put letters next to each other.
'utter' after 'c' is 'CUTTER'.
Can you help me to learn more?
(Other definitions for cutter that I've seen before include "Boat; tailor" , "Ship's boat" , "Fast patrol boat" , "One working with Ruby possibly" , "Vessel (but not a clipper!)" .)