Man going half a mile in a vessel (5)
I believe the answer is:
'man going' is the definition.
'valet' can be an answer for 'man' (I've seen this in another clue). I am not sure about the 'going' bit.
'half a mile in a vessel' is the wordplay.
'half' suggests halving the letters.
'in' is an insertion indicator.
'a vessel' becomes 'vat' (vat is a kind of vessel).
'mile' cut in half is 'le'.
'le' put into 'vat' is 'VALET'.
Can you help me to learn more?
(Other definitions for valet that I've seen before include "Clean inside" , "Gentleman%u2019s gentleman" , "Batman" , "Personal serving man" , "Gentleman's gentleman" .)