Old Jack's vessel (5)
I believe the answer is:
ketch
This clue could be a double definition.
'old jack's' is the first definition.
'ketch' can be an answer for 'jack's' (I've seen this before). I'm unsure of the 'old' bit.
'vessel?' is the second definition.
(ketch is a kind of vessel)
Can you help me to learn more?
(Other definitions for ketch that I've seen before include "Two-masted sailing boat" , "Two-masted sailing ship" , "Jack" , "Small sailing vessel with two masts" , "Two-masted sailing vessel" .)