A challenging read: fans weakening terribly! (9,4)

I believe the answer is:

finnegans wake

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

'fans weakening terribly' is the wordplay.
'terribly' indicates an anagram.
'fans'+'weakening'='fansweakening'
'fansweakening' is an anagram of 'FINNEGANS WAKE'.

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(Other definitions for finnegans wake that I've seen before include "book title" , "Knew insane fag (anag) -- novel by 5" , "Book by James Joyce" , "Irish achievement" , "James Joyce novel" .)

I've seen this clue in The Telegraph.

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