One outcome of simple test for sucker? (4,4)

I believe the answer is:

easy mark

'sucker?' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

(Other definitions for easy mark that I've seen before include "Something easy to hit" , "Person simple to beat or fool" , "Sitting duck, defenceless victim" , "Gull" .)

I've seen this clue in The Guardian.

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