... "a twat essentially with too much capital" (6)
I believe the answer is:
ottawa
'capital' is the definition.
(capital of Canada)
'a twat essentially with too much' is the wordplay.
'essentially' says to take the centre.
'with' means one lot of letters go next to another.
'too much' becomes 'OTT' (short for over the top).
The middle letters of 'twat' are 'wa'.
'a'+'wa'='awa'
'awa' after 'ott' is 'OTTAWA'.
(Other definitions for ottawa that I've seen before include "N. American city" , "North American city" , "Canada's capital" , "Canadian capital" , "Canadian city" .)