What might secure bottom, but not one side (7)
I believe the answer is:
Here is my best explanation:
'one side' is the definition.
(English football club or side)
I don't understand the remainder of the clue.
(Other definitions for arsenal that I've seen before include "Mil. magazine; FA club", "Military store for arms, ammunition etc", "Armoury - London football club", "array of weapons", "supply", "London FC", "Depot for arms and other military equipment", "Where the guns are and who are The Gunners are", "team effort" and "Football team - magazine".)