Floating platform right at the back (4)
I believe the answer is:
raft
'floating platform' is the definition.
(I've seen this in another clue)
'right at the back' is the wordplay.
'right' becomes 'r' (common abbreviation).
'at' is a charade indicator (letters next to each other).
'the back' becomes 'aft' (at the back of a ship).
'r'+'aft'='RAFT'
(Other definitions for raft that I've seen before include "Flat structure for travel over water" , "Simple craft" , "Floating framework" , "host" , "River transport" .)