Lethargy of Politburo put Stalin in the ascendant (6)
I believe the answer is:
stupor
'lethargy' is the definition.
(I've seen this before)
'politburo put stalin in the ascendant' is the wordplay.
'in' indicates the answer is hidden within the clue.
'the ascendant' is a reversal indicator.
'roputs' is hidden in the letters of 'politburo put stalin'.
'roputs' written backwards gives 'STUPOR'.
'of' is the link.
(Other definitions for stupor that I've seen before include "lethargic" , "numbness" , "Stupefaction, daze" , "Dazed state; Proust (anag.)" , "Almost unconscious state" .)