The vessel's left behind half of us - what fun! (5)
I believe the answer is:
sport
'fun' is the definition.
(I know that fun can be written as sport)
'the vessel's left behind half of us' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'vessel' becomes 'rop' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'left' is a reversal indicator.
'behind' means one lot of letters go next to another.
'half of' means to take half the letters.
't'+'rop'='trop'
'trop' back-to-front is 'port'.
'us' with half the letters taken is 's'.
'port' put after 's' is 'SPORT'.
'what' acts as a link.
Can you help me to learn more?
(Other definitions for sport that I've seen before include "Physical activity" , "Show off" , "Eg, cricket" , "Physical exertion and competition" , "entertainment" .)