Leftists in pocket, perhaps? (4)

I believe the answer is:

reds

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'leftists' is the definition.
(red is a kind of leftist)

I cannot understand how the remainder of the clue works.

(Other definitions for reds that I've seen before include "These are not right" , "Colours for communists" , "Tidies up" , "Scarlet communists" , "Left-wingers" .)

I've seen this clue in the Evening Standard.

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