Lethargy could be Proust's undoing (6)
I believe the answer is:
stupor
'lethargy' is the definition.
(I've seen this in another clue)
'proust's undoing' is the wordplay.
'undoing' is an anagram indicator.
'proust' is an anagram of 'STUPOR'.
'could be' acts as a link.
(Other definitions for stupor that I've seen before include "State of insensibility or near-unconsciousness" , "Shock - Proust (anag)" , "Suspension of sensibility" , "Insensible state" , "State of senselessness" .)