A number of us scowl (4)

I believe the answer is:

lour

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'scowl' is the definition.
(lour can mean a scowl or frown)

'a number of us' is the wordplay.
'a number' becomes 'L' (fifty in Roman numerals).
'of' becomes 'o''.
'us' becomes 'ur' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'l'+'o'+'ur'='LOUR'

Can you help me to learn more?

(Other definitions for lour that I've seen before include "(Of sky) look dark" , "Look sullen - frown" , "Threaten" , "Threatening appearance" , "Scowl or look angry" .)

I've seen this clue in The Globe and Mail.

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