Leading sailor slips into vessels, brags (6)
I believe the answer is:
boasts
'brags' is the definition.
('boast' can be a synonym of 'brag')
'leading sailor slips into vessels' is the wordplay.
'leading sailor slips' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'into' is an insertion indicator.
'vessels' becomes 'boats' (boat is a kind of vessel).
's' going into 'boats' is 'BOASTS'.
Can you help me to learn more?
(Other definitions for boasts that I've seen before include "crows" , "Blows one's own trumpet" , "Brags, blows" , "Brags, shows off" , "Brags in praise of oneself" .)