Top after game advanced position (10)
I believe the answer is:
bridgehead
'advanced position' is the definition.
(I've seen this before)
'top after game' is the wordplay.
'top' becomes 'head' (head is a kind of top).
'after' means one lot of letters go next to another.
'game' becomes 'bridge' (bridge is a kind of game**).
'head' put after 'bridge' is 'BRIDGEHEAD'.
(Other definitions for bridgehead that I've seen before include "Forward position seized by attacking troops" , "Strong position secured inside enemy territory" , "Advantageous position" , "defensive position" , "Advanced position secured in enemy territory" .)