Crossword Genius

Mould stum? (4)

Ross

I believe the answer is:

must

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

'mould stum?' is the definition.
'must' can be an answer for 'mould' (I've seen this before). I am unsure of the 'stum?' bit.

'mould stum?' is the wordplay.
'mould' indicates anagramming the letters.
'stum' anagrammed gives 'MUST'.

Can you help me to learn more?

(Other definitions for must that I've seen before include "Mould - grape juice during fermentation" , "Essential to have" , "Prerequisite" , "Unfermented wine" , "The mould is compelled" .)

I've seen this clue in The Independent.
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