A power unit rejected by old capital city (6)
I believe the answer is:
ottawa
'capital city' is the definition.
(I've seen this before)
'a power unit rejected by old' is the wordplay.
'power unit' becomes 'watt' (unit of power).
'rejected' says the letters should be written backwards.
'by' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'a'+'watt'='awatt'
'awatt' in reverse letter order is 'ttawa'.
'ttawa' put after 'o' is 'OTTAWA'.
(Other definitions for ottawa that I've seen before include "Foreign capital" , "Canadian capital" , "capital city" , "N. American city" , "somewhere in Canada" .)