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 can't explain the remainder of the clue.
(Other definitions for arsenal that I've seen before include "Arms and ammunition store", "Mil. magazine; FA club", "First FA Cup winners", "London football club - magazine", "Ammunition store or English soccer team", "Store of weapons or English soccer team.", "Where the guns are and who are The Gunners are", "All a country's weapons", "A military store of guns and armaments" and "Weaponry".)