Mum showed great enthusiasm when given one old coin (8)

'old coin' is the definition.
The definition suggests a singular noun which matches the answer.

'mum showed great enthusiasm when given one' is the wordplay.
I cannot really understand how this works, but
'mum' could be 'ma' (both can mean mother) and 'ma' is found within the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
The remaining letters 'raved' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?