Vessel foremost for cargo, say (6)
I believe the answer is:
cutter
'vessel' is the definition.
(cutter is a kind of vessel)
'foremost for cargo say' is the wordplay.
'foremost for' indicates taking the first letters.
'say' becomes 'utter' (to utter something is to speak or say it).
The first letter of 'cargo' is 'c'.
'c'+'utter'='CUTTER'
(Other definitions for cutter that I've seen before include "Sailing vessel" , "E.g. knife" , "Boat; tailor" , "Slicing tool of fast sailing ship" , "One working with Ruby possibly" .)