Queen of the witches can't stand going round the City (6)
I believe the answer is:
hecate
'queen of the witches' is the definition.
The definition suggests a singular noun which matches the answer.
'can't stand going round the city' is the wordplay.
'can't stand' becomes 'hate' (if you hate something you can't stand it).
'going round' is an insertion indicator (some letters go around others).
'the city' becomes 'EC' (postcode for the City of London).
'hate' going around 'ec' is 'HECATE'.
Can you help me to learn more?
(Other definitions for hecate that I've seen before include "Greek goddess of the Underworld" , "Greek goddess of witchcraft and ghosts" , "evil witch" , "Ancient goddess found in the ace" , "Female with spells" .)