Checks off blood suckers (5)

I believe the answer is:

ticks

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'checks' is the definition.
(I know this)

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

(Other definitions for ticks that I've seen before include "Marks of approval" , "Mites in mattress" , "Makes noise like old clock" , "They suck from warm-blooded animals" , "covers" .)

I've seen this clue in the Sydney Morning Herald.

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