Formed core of weapon in cast (6)
I believe the answer is:
shaped
'formed' is the definition.
('shape' can be a synonym of 'form')
'core of weapon in cast' is the wordplay.
'core of' says to take the centre.
'weapon' becomes 'BB shot' (BB shot is an example).
'in cast' becomes 'aped' (I can't justify this - if you can you should give a lot more credence to this answer).
The centre of 'bbshot' is 'sh'.
'sh'+'aped'='SHAPED'
Can you help me to learn more?
(Other definitions for shaped that I've seen before include "modelled" , "Form-filling" , "Formed or fashioned" , "Sculpted" , "Moulded, wrought" .)