Duke is fellow endlessly sad (6)

I believe the answer is:

dismal

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'sad' is the definition.
(thesaurus)

'duke is fellow endlessly' is the wordplay.
'duke' becomes 'd'.
'fellow endlessly' becomes 'mal' (I can't justify this - if you can you should believe this answer much more).
'd'+'is'+'mal'='DISMAL'

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(Other definitions for dismal that I've seen before include "Depressing in character or appearance" , "Gloomy and depressing like winter weather" , "Gloomy, dreary" , "Wretched" , "Pessimist -- is my jam mild? (anag)" .)

I've seen this clue in The Times.

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