Ordinary power returns to a capital (6)
I believe the answer is:
ottawa
'capital' is the definition.
(Ottawa is an example)
'ordinary power returns to a' is the wordplay.
'ordinary' becomes 'o' (abbreviation e.g. O-Levels).
'power' becomes 'watt'.
'returns' says the letters should be written backwards.
'watt' in reverse letter order is 'ttaw'.
'o'+'ttaw'+'a'='OTTAWA'
(Other definitions for ottawa that I've seen before include "Canadian capital" , "North American city" , "N. American city" , "The capital of Canada (6)" , "Canada's capital" .)