Shorten Arsenal’s opening game (7)
I believe the answer is:
abridge
'shorten' is the definition.
('abridge' can be a synonym of 'shorten')
'arsenal's opening game' is the wordplay.
'opening' says to take the initial letters.
'game' becomes 'bridge' (bridge is a kind of game).
The initial letter of 'arsenals' is 'a'.
'a'+'bridge'='ABRIDGE'
(Other definitions for abridge that I've seen before include "Cut short" , "(US) cattle-pen" , "Abbreviate, contract" , "Remove sections" , "Edit down" .)