Concentrates while taking in game (8)
I believe the answer is:
abridges
'concentrates' is the definition.
(concentrate can mean to abridge or reduce down)
'while taking in game' is the wordplay.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'taking in' means one lot of letters goes inside another.
'game' becomes 'bridge' (card game).
'as' placed around 'bridge' is 'ABRIDGES'.
(Other definitions for abridges that I've seen before include "Shortens" , "Makes short work of" .)