Crossword Genius

Unions alone must become reformed, but not at the same time (15)

Ross

I believe the answer is:

nonsimultaneous

I'm a little stuck... Click here to teach me more about this clue!

'reformed but not at the same time' is the definition.
I can't tell whether this definition defines the answer.

'unions alone must become' is the wordplay.
'become' is an anagram indicator.
'unions'+'alone'+'must'='unionsalonemust'
'unionsalonemust' is an anagram of 'NONSIMULTANEOUS'.

Can you help me to learn more?

I've seen this clue in The Guardian.
Want a hint initially instead of a full solution? Install my app