Vessel right at the back (4)
I believe the answer is:
raft
'vessel' is the definition.
(I've seen this before)
'right at the back' is the wordplay.
'right' becomes 'r' (abbreviation).
'at' says to put letters next to each other.
'the back' becomes 'aft' (I've seen this in other clues).
'r'+'aft'='RAFT'
(Other definitions for raft that I've seen before include "Makeshift boat" , "Structure made of timber" , "Simple floating structure" , "Flat, floating platform" , "Flat floating structure used as boat" .)