County council’s final advantage, supplying ballast (9)
I believe the answer is:
kentledge
'ballast' is the definition.
(kentledge is a type of ballast)
'county council's final advantage' is the wordplay.
'county' becomes 'kent' (Kent is an example).
'final' suggests the final letters.
'advantage' becomes 'edge' (having the edge is having an advantage).
The last letter of 'council' is 'l'.
'kent'+'l'+'edge'='KENTLEDGE'
'supplying' acts as a link.
(Other definitions for kentledge that I've seen before include "iron in vessel" , "Ship's ballast" .)