Seat at the back (6)
I believe the answer is:
behind
I believe this is a double definition.
'seat' is the first definition.
(I know that seat can be written as behind)
'the back' is the second definition.
(thesaurus)
'at' is the link.
(Other definitions for behind that I've seen before include "bum" , "Seat" , "Serving time" , "Hidden by; bottom" , "Hidden by; late" .)