Beats correct marks

I believe the answer is:

ticks

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'beats' is the definition.
('tick' can be a synonym of 'beat')

'correct marks' is the wordplay.
'correct' becomes 'tick' (a tick is used to mark an answer correct).
'marks' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'tick'+'s'='TICKS'

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(Other definitions for ticks that I've seen before include "Marks correct" , "They suck from warm-blooded animals" , "Parasitic mites" , "Parasites" , "Mites in mattress" .)

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

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