It's usually written to one unable to read it (7)

I believe the answer is:

epitaph

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'it' is the definition.
The definition suggests a singular noun which matches the answer.

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

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(Other definitions for epitaph that I've seen before include "message about late departure" , "Gravestone inscription" , "Tombstone inscription" , "Words carved on a gravestone" , "Pit heap (anag)" .)

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

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