Clean vessel boarded by new leader (5)
I believe the answer is:
scrub
'clean' is the definition.
(scrub can mean to clean)
'vessel boarded by new leader' is the wordplay.
'vessel' becomes 'sub' (submarine is a type of vessel).
'boarded by' means one lot of letters goes inside another (I've seen this in other clues).
'new leader' becomes 'cr' (I can't justify this - if you can you should give a lot more credence to this answer).
'sub' placed around 'cr' is 'SCRUB'.
Can you help me to learn more?
(Other definitions for scrub that I've seen before include "Eradicate" , "Wipe out" , "Wash the stunted shrub?" , "Wash vigorously" , "Drop" .)