How a boxer might read poem on the radio - its lines are, crucially, of different sizes (7)

'crucially of different sizes' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'how a boxer might read poem on the radio its lines' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'poem' could be 'ode' (ode is a kind of poem) and 'ode' is present in the answer.
'lines' could be 'br' and 'br' is found in the remaining letters.
A single letter 'c' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...

'are' acts as a link.

Can you help me to learn more?