Crossword Genius

Often Mum's describing an unknown era (4,1,4)

Ross

I believe the answer is:

many a time

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

'often' is the definition.
(thesaurus)

'mum's describing an unknown era' is the wordplay.
I cannot quite understand how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is located in the answer.
'unknown' could be 'y' (algebra) and 'y' is found within the answer.
'mum' could be 'ma' ('ma' can be a synonym of 'mum') and 'ma' is present in the answer.
This explanation may well be incorrect...

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I've seen this clue in The Guardian.
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