Essential part — ways to come up with a magazine (7)
I believe the answer is:
arsenal
'magazine' is the definition.
(I've seen this before)
'essential part ways to come up with a' is the wordplay.
I cannot quite see how this works, but
'part' could be 'arse' (arse is a kind of part) and 'arse' is found in the answer.
'a' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for arsenal that I've seen before include "Store of guns and ammunition" , "Military store for arms, ammunition etc" , "Depot for arms and other military equipment" , "Supply of weapons" , "Armoury - London football club" .)