Group only look up occasionally to scowl (4)

I believe the answer is:

lour

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

I can't explain the remainder of the clue.

(Other definitions for lour that I've seen before include "Threatening appearance" , "Frown threateningly" , "Look sullen - frown" , "Scowl or look angry" , "Scowl, look dark and threatening" .)

I've seen this clue in The Irish Times.

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