Investigates The Last Word over cover-up (6)

I believe the answer is:

probes

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'investigates' is the definition.
(probing is a kind of investigating)

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

Can you help me to learn more?

(Other definitions for probes that I've seen before include "Conducts penetrating investigation" , "Examines in depth as dentist does tooth" , "Questions or examines thoroughly" , "Examines carefully as dentist does a bad tooth" , "Investigates as dentist does tooth" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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