Council beginning to organise contest to fill vacated seat (6)
I believe the answer is:
soviet
'council' is the definition.
(soviet is a kind of council)
'beginning to organise contest to fill vacated seat' is the wordplay.
'beginning to' suggests taking the first letters.
'contest' becomes 'vie' (I've seen this before**).
'to fill' indicates putting letters inside.
'vacated' suggests removing the centre.
'seat' with its centre taken out is 'st'.
The first letter of 'organise' is 'o'.
'o'+'vie'='ovie'
'ovie' placed into 'st' is 'SOVIET'.
(Other definitions for soviet that I've seen before include "Old Russian council" , "Union of ... Socialist Republics" , "Elected council in former USSR" , "Old government council" , "National council in former USSR" .)