Seat in the upper chamber (5)
I believe the answer is:
perch
'seat' is the definition.
(perch is a kind of seat)
'in the upper chamber' is the wordplay.
'in' indicates the answer is hidden within the clue.
'PERCH' is hidden in the letters of 'upper chamber'.
(Other definitions for perch that I've seen before include "Bird's seat - fish" , "Fish and imperial measure" , "Freshwater game fish" , "Fish; pole" , "[SOMETHING FISHY]" .)