Birmingham maybe securing a victory, initially hollow (6)
I believe the answer is:
cavity
'hollow' is the definition.
(I know that cavity is a type of hollow)
'birmingham maybe securing a victory initially' is the wordplay.
'birmingham maybe' becomes 'city' (Birmingham is a city).
'securing' means one lot of letters goes inside another.
'initially' says to take the initial letters.
The first letter of 'victory' is 'v'.
'a'+'v'='av'
'city' enclosing 'av' is 'CAVITY'.
(Other definitions for cavity that I've seen before include "Hole, crater" , "Hollow space" , "Hole, gap" , "A hole could mean work for the dentist" , "Decayed part of tooth" .)