Apprehends about a thousand supporters (3-5)

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armrests

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'apprehends about a thousand supporters' is the definition.
I can't judge whether this definition defines the answer.

'apprehends about a thousand supporters' is the wordplay.
'apprehends' becomes 'arrests' ('arrest' can be a synonym of 'apprehend').
'about' means one lot of letters goes inside another.
'a thousand supporters' becomes 'm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'arrests' going around 'm' is 'ARMRESTS'.

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(Other definitions for armrests that I've seen before include "supports" , "Side parts of a chair" .)

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

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