Horrify a friend, seizing power (5)
I believe the answer is:
appal
'horrify' is the definition.
('appal' can be a synonym of 'horrify')
'a friend seizing power' is the wordplay.
'friend' becomes 'pal' (pal is a kind of friend**).
'seizing' means one lot of letters goes inside another.
'power' becomes 'p'.
'pal' going around 'p' is 'ppal'.
'a'+'ppal'='APPAL'
(Other definitions for appal that I've seen before include "Fill with horror" , "Petrify" , "Put at rest (suspicion)" , "expressed dismay" , "Dismay, horrify" .)