Vessel in snake pit at last (4)
I believe the answer is:
boat
'vessel' is the definition.
(boat is a kind of vessel)
'snake pit at last' is the wordplay.
'snake' becomes 'boa' (boa is a kind of snake**).
'at last' says to take the final letters.
The last letter of 'pit' is 't'.
'boa'+'t'='BOAT'
'in' acts as a link.
(Other definitions for boat that I've seen before include "Vehicle for travelling over water" , "Vessel for travelling on water" , "Small vessel" , "Small ship" , "Water transport" .)