Back a certain amount of power with zero capital (6)
I believe the answer is:
ottawa
'capital' is the definition.
(capital of Canada)
'back a certain amount of power with zero' is the wordplay.
'back' shows that the letters should be reversed in order.
'certain amount of power' becomes 'watt' (unit of power).
'with' means one lot of letters go next to another.
'zero' becomes 'O' (O resembles 0).
'a'+'watt'='awatt'
'awatt' reversed gives 'ttawa'.
'ttawa' after 'o' is 'OTTAWA'.
(Other definitions for ottawa that I've seen before include "capital city" , "Canadian capital" , "Canadian city" , "somewhere in Canada" , "Canada's capital" .)