Third book an unlikely success, opening being omitted (4)

I believe the answer is:

luke

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'third' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's a link between them I don't understand?

I don't understand the remainder of the clue.

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(Other definitions for luke that I've seen before include "Third NT book" , "Patron saint of physicians" , "Biblical evangelist" , "Matthew, Mark, - - and John" , "Gospel writer" .)

I've seen this clue in The Independent.

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