Abandoned area separating two sets of players (4,5)
I believe the answer is:
cast aside
'abandoned' is the definition.
'area separating two sets of players' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'separating' means one lot of letters goes inside another.
'two sets of' becomes 'cast' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'players' becomes 'side' (I've seen this before).
'cast'+'side'='castside'
'a' placed within 'castside' is 'CAST ASIDE'.
(Other definitions for cast aside that I've seen before include "Ditch" , "dropped" , "Get rid of" .)