In city regularly banning butter (5)
I believe the answer is:
nanny
'butter' is the definition.
(I've seen this before)
'in city regularly banning' is the wordplay.
'in' means one lot of letters goes inside another.
'city' becomes 'ny' (I've seen this in other clues**).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'banning' are 'ann'.
'ny' placed around 'ann' is 'NANNY'.
(Other definitions for nanny that I've seen before include "Carer for children" , "Overprotective government deplored by Thatcher" , "Issue tender" , "Children's nurse" , "Person minding another's child" .)