One among Queen Victoria's first back in capital (6)
I believe the answer is:
vienna
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'among queen victoria's first back in capital' is the wordplay.
'among' is an insertion indicator (I've seen this in other clues).
'queen' becomes 'anne' (I've seen this before**).
'first' suggests taking the first letters.
'back' is a reversal indicator.
'capital' indicates taking the first letters.
The initial letter of 'victoria' is 'v'.
The first letter of 'in' is 'i'.
'anne'+'v'='annev'
'annev' backwards is 'venna'.
'venna' placed around 'i' is 'VIENNA'.
(Other definitions for vienna that I've seen before include "Capital city on the Danube" , "European city for New Year's Day concert" , "Austrian city of the waltz" , "somewhere in Austria" , "sort of sausage" .)