Clean vessel with king aboard (5)
I believe the answer is:
scrub
'clean' is the definition.
(scrub can mean to clean)
'vessel with king aboard' is the wordplay.
'vessel with' becomes 'sub' (I can't explain this - if you can you should give a lot more credence to this answer).
'king' becomes 'CR' (short for Carolus Rex, King Charles in Latin).
'aboard' means one lot of letters goes inside another.
'sub' placed around 'cr' is 'SCRUB'.
Can you help me to learn more?
(Other definitions for scrub that I've seen before include "plants" , "Abandon" , "Scour; cancel" , "Wash with a brush" , "Wipe out" .)