Power possessed by mysterious secret ceremonial staff (7)
I believe the answer is:
sceptre
'ceremonial staff' is the definition.
(I've seen this before)
'power possessed by mysterious secret' is the wordplay.
'power' becomes 'p'.
'possessed by' indicates putting letters inside.
'mysterious' indicates an anagram.
'secret' is an anagram of 'scetre'.
'p' going into 'scetre' is 'SCEPTRE'.
(Other definitions for sceptre that I've seen before include "Respect (anag) -- symbol of ruler's authority" , "Ruler's staff" , "Staff of regency" , "Emblem of royalty" , "Royal baton" .)