Seat in upper chamber (5)
I believe the answer is:
perch
'seat' is the definition.
(perch is a kind of seat)
'in upper chamber' is the wordplay.
'in' indicates the answer is hidden within the clue.
'PERCH' is hidden within 'upper chamber'.
(Other definitions for perch that I've seen before include "Rest (as a bird)" , "Roosting place" , "A fish; alighting place" , "Roost; old measure" , "Freshwater fish - roost" .)