Ring a power unit withdrawing capital (6)
I believe the answer is:
ottawa
'capital' is the definition.
(capital of Canada)
'ring a power unit withdrawing' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'power unit' becomes 'watt' (unit of power).
'withdrawing' says the letters should be written backwards.
'a'+'watt'='awatt'
'awatt' back-to-front is 'ttawa'.
'o'+'ttawa'='OTTAWA'
(Other definitions for ottawa that I've seen before include "capital city" , "somewhere in Canada" , "The capital of Canada (6)" , "Canada's capital" , "N. American city" .)