Mother cast in capital part? That's a long story (10)

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

'cast in capital part? that's a long story' is the wordplay.
'cast in' indicates putting letters inside.
'capital' becomes 'Riga' (capital of Latvia).
'part?' becomes 'role' (eg a theatrical role).
'that's a long story' becomes 'am' (I can't justify this - if you can you should give a lot more credence to this answer).
'riga'+'role'='rigarole'
'rigarole' going around 'am' is 'RIGAMAROLE'.

Can you help me to learn more?