Servant going half a mile in a vessel (5)
I believe the answer is:
'servant' is the definition.
(I know that valet is a type of house servant)
'half a mile in a vessel' is the wordplay.
'half a' suggests halving the letters.
'in' means one lot of letters goes inside another.
'a vessel' becomes 'vat' (vat is a kind of vessel).
'mile' with half the letters taken is 'le'.
'le' put within 'vat' is 'VALET'.
'going' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for valet that I've seen before include "dresser" , "Batman" , "Personal male assistant" , "'No man is a hero to his . . . . .'" , "Gentleman%u2019s gentleman" .)