Floating structure right behind (4)
I believe the answer is:
raft
'floating structure' is the definition.
(I've seen this before)
'right behind' is the wordplay.
'right' becomes 'r' (common abbreviation).
'behind' becomes 'aft' (aft means towards or at the end of a ship).
'r'+'aft'='RAFT'
(Other definitions for raft that I've seen before include "Structure that floats" , "Wooden craft" , "Simple floating structure" , "Simple boat" , "water transport" .)