Mug with drink for old copper (6)

I believe the answer is:

cuprum

I'm a little stuck... Click here to teach me more about this clue!

'old copper' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'mug with drink' is the wordplay.
'mug' becomes 'cup'.
'with' says to put letters next to each other.
'drink' becomes 'rum' (rum is a kind of drink).
'cup'+'rum'='CUPRUM'

'for' acts as a link.

Can you help me to learn more?

Want a hint initially instead of a full solution? Install my app