the ellipsis of verb in a complex sentence

[Iwanna]

Future historians will be in a unique position when they come to record the history of our own times. They will hardly know which facts to select from the great mass of evidence that steadily accumulates. What is more, they will not have to rely solely on the written word. Films, videos, CDs and CD-ROMs are just some of the bewildering amount of information they will have. They will be able, as it were, to see and hear us in action. But the historian attempting to reconstruct the distant past is always faced with a difficult task. He has to deduce what he can from the few scanty clues available.

In the last sentence, the verb is left out. Usually, as I have been taught, the omitted verb is the same one as previously occurs(here, the "deduce"). But it seems obscure to me if "deduce" is placed there. What does this sentence exactly mean?

thanks

[Mister Micawber]

.
He has to deduce what (= as much as) he can [deduce] from the available clues.
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Native English teacher at Mister Micawber's