Gentleman with power rising in authentic act of revenge (8)
I believe the answer is:
reprisal
'act of revenge' is the definition.
(I've seen this before)
'gentleman with power rising in authentic' is the wordplay.
'gentleman' becomes 'sir' (I've seen this before).
'with power' becomes 'p' (I am not sure about the 'with' bit.**).
'rising' says the letters should be written backwards (going up in a down clue).
'in' indicates putting letters inside.
'authentic' becomes 'real' (I've seen this before).
'sir'+'p'='sirp'
'sirp' backwards is 'pris'.
'pris' placed into 'real' is 'REPRISAL'.
(Other definitions for reprisal that I've seen before include "Retaliatory action against an enemy" , "hitting back" , "compensation" , "Act of revenge" , "revenge attack" .)