Lethargy produced by Proust novel (6)
I believe the answer is:
stupor
'lethargy' is the definition.
(thesaurus)
'proust novel' is the wordplay.
'novel' is an anagram indicator.
'proust' anagrammed gives 'STUPOR'.
'produced by' is the link.
(Other definitions for stupor that I've seen before include "result of too much snow?" , "State of senselessness" , "Proust found in dazed condition" , "State of being stunned" , "mental dullness?" .)