There's nothing for me to call round for but the medicine (7)

'the medicine' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'there's nothing for me to call round for' is the wordplay.
I cannot quite understand how this works, but
'nothing' could be 'o' (looks like zero - 0) and 'o' is present in the answer.
'me' is within the answer.
'to call' could be 'call' and 'call' is found in the remaining letters.
No letters remain.
This explanation may well be incorrect...

'but' is the link.

Can you help me to learn more?