Revolutionary purge at an all-time low (5)
I believe the answer is:
nadir
'all-time low' is the definition.
(I've seen this before)
'revolutionary purge at an' is the wordplay.
'revolutionary' says the letters should be written in reverse.
'purge' becomes 'rid' (I've seen this in other clues).
'at' says to put letters next to each other.
'rid'+'an'='ridan'
'ridan' backwards is 'NADIR'.
(Other definitions for nadir that I've seen before include "Drain (anag)" , "Rock bottom" , "pits" , "It's the depths" , "Very bottom, farthest from zenith" .)