Saw the head for examination purposes? (6)

I believe the answer is:

trepan

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'saw' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for trepan that I've seen before include "Cranial surgical instrument" , "Perforate skull (med.)" , "Make perforation in person's skull" , "old saw" , "Surgically create a hole in the skull" .)

I've seen this clue in The Globe and Mail.

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