They make trouble in rows or tiers perhaps (7)

I believe the answer is:

rioters

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'they make trouble in rows' is the definition.
The definition suggests an adverb but the answer is not.

'or tiers perhaps' is the wordplay.
'perhaps' indicates anagramming the letters.
'or'+'tiers'='ortiers'
'ortiers' anagrammed gives 'RIOTERS'.

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(Other definitions for rioters that I've seen before include "Violent protesters" , "Rabble-rousers" , "Violent troublemakers" , "They disturb the peace" , "Crowd disturbing the peace" .)

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

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