A dieter regularly visiting capital city up north (8)
I believe the answer is:
aberdeen
'city up north' is the definition.
(I've seen this before)
'a dieter regularly visiting capital' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'visiting' is an insertion indicator.
'capital' becomes 'bern' (capital of Switzerland).
The alternate letters of 'dieter' are 'dee'.
'dee' inserted into 'bern' is 'berdeen'.
'a'+'berdeen'='ABERDEEN'
(Other definitions for aberdeen that I've seen before include "Scotland's Granite City" , "Scottish port" , "____ Angus, a black hornless cattle breed" , "Scottish university" , "Scottish East coast city" .)