Does it always make the straphanger laugh? (8,4)

I believe the answer is:

standing joke

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'laugh?' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for standing joke that I've seen before include "laugh" , "Regular cause of amusement" , "Regular source of amusement or ridicule" , "Constant source of amusement" .)

I've seen this clue in the Evening Standard.

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