Old measure of power brought back by a Canadian city (6)
I believe the answer is:
ottawa
'canadian city' is the definition.
(I've seen this before)
'old measure of power brought back by a' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'measure of power' becomes 'watt' (unit of power).
'brought back' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'watt' back-to-front is 'ttaw'.
'o'+'ttaw'+'a'='OTTAWA'
(Other definitions for ottawa that I've seen before include "Canadian capital" , "Canadian city" , "The capital of Canada (6)" , "somewhere in Canada" , "N. American city" .)