Power seized by wickedly secret ghost (7)
I believe the answer is:
spectre
'ghost' is the definition.
(I know that ghost can be written as spectre)
'power seized by wickedly secret' is the wordplay.
'power' becomes 'p' (symbol in physics).
'seized by' means one lot of letters goes inside another.
'wickedly' is an anagram indicator (I've seen this in other clues).
'secret' anagrammed gives 'sectre'.
'p' placed inside 'sectre' is 'SPECTRE'.
(Other definitions for spectre that I've seen before include "Sceptre (anag.)" , "Ghost, phantom, apparition" , "Enemy organisation for James Bond; ghost" , "Narrow ravine" , "Respect for a ghost" .)