Shorten Arsenal's first game (7)
I believe the answer is:
abridge
'shorten' is the definition.
(synonyms)
'arsenal's first game' is the wordplay.
'arsenal's first' becomes 'a' (1st letter of 'arsenal').
'game' becomes 'bridge' (card game).
'a'+'bridge'='ABRIDGE'
(Other definitions for abridge that I've seen before include "Reduce the length of" , "Make short work" , "Shorten by using fewer words" , "Shorten (eg, text)" , "Make shorter, book say" .)