Is unable to write long song (5)

I believe the answer is:

canto

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'song' is the definition.
(thesaurus)

'is unable to write long' is the wordplay.
'is unable to' becomes 'can't' (can't).
'write long' becomes 'o' (I am not sure about this - if you are sure you should believe this answer much more).
'cant'+'o'='CANTO'

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(Other definitions for canto that I've seen before include "Major division of a long poem" , "Melody in choral music" , "part of long poem" , "Main division of long poem" , "melodic piece of music" .)

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

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