One off to see Musicians Union over failure, after note's missed (11)

'one' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'to see musicians union over failure after note's missed' is the wordplay.
I cannot quite see how this works, but
'see' could be 'c' (the word for the letter, according to Chambers) and 'c' is found within the answer.
'over' could be 'oer' (synonyms) and 'oer' is located in the answer.
'after' could be 'once' (once can mean immediately after) and 'once' is present in the answer.
'note' could be 'g' (musical note) and 'g' is found in the answer.
This explanation may well be incorrect...

'off' is the link.

Can you help me to learn more?