Essay I set out to support X tenderfoot carried (8)

I believe the answer is:

treatise

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

I cannot understand how the rest of the clue works.

(Other definitions for treatise that I've seen before include "Learned work on particular subject" , "formal document" , "Systematically-written formal piece" , "Thesis" , "Formal piece of writing on a particular subject" .)

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

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