Composer's mass I know rewritten across unknown years (11)

'composer's' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's a link between them I don't understand?

'mass i know rewritten across unknown years' is the wordplay.
I cannot really understand how this works, but
'mass' could be 'm' (symbol used in Physics e.g. f=ma) and 'm' is found within the answer.
'i' could be 'a' and 'a' is located in the answer.
'unknown' could be 'z' (algebra) and 'z' is found within the answer.
'years' could be 'y' (abbreviation) and 'y' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?