Defence in Arsenal's opening games in division (7)
I believe the answer is:
parapet
'defence' is the definition.
(parapet is a kind of defence)
'arsenal's opening games in division' is the wordplay.
'opening' indicates taking the first letters.
'games' becomes 'pe' (abbreviation for Physical Education).
'in' indicates putting letters inside.
'division' becomes 'part' (synonyms).
The first letter of 'arsenals' is 'a'.
'a'+'pe'='ape'
'ape' inserted within 'part' is 'PARAPET'.
'in' is the link.
(Other definitions for parapet that I've seen before include "low bank" , "balustrade" , "form of protection" , "Low barrier edging balcony" , "keep overlooking this?" .)