Power is money, a discontented tyrant's admitted (7)
I believe the answer is:
wattage
'power' is the definition.
(I know that wattage is a type of power)
'money a discontented tyrant's admitted' is the wordplay.
'money' becomes 'wage' (I've seen this before).
'discontented' suggests removing the centre (the word's contents are removed).
'admitted' is an insertion indicator (inserted letters are admitted in).
'tyrant' with its middle taken out is 'tt'.
'a'+'tt'='att'
'wage' going around 'att' is 'WATTAGE'.
'is' acts as a link.
(Other definitions for wattage that I've seen before include "Amount of electrical power" , "Electrical power rating" , "Light bulb power rating" .)