Football club left behind sporting arenas (7)
I believe the answer is:
arsenal
'football club' is the definition.
(Arsenal football club)
'left behind sporting arenas' is the wordplay.
'left' becomes 'L' (common abbreviation).
'behind' means one lot of letters go next to another.
'sporting' is an anagram indicator.
'arenas' anagrammed gives 'arsena'.
'l' after 'arsena' is 'ARSENAL'.
(Other definitions for arsenal that I've seen before include "Store of weapons or English soccer team." , "English football club and military depot" , "Munitions store and English football team" , "London FC" , "Weapon store - team" .)