Dirk's tome has no headings. How tedious! (7)

I believe the answer is:

irksome

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'tedious' is the definition.
(I know that tedious can be written as irksome)

I can't explain the rest of the clue.

(Other definitions for irksome that I've seen before include "Troublesome, vexing" , "Smoker I find annoying" , "'Tedious, tiresome (7)'" , "Annoying or tiresome" , "Burdensome and tedious" .)

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

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