Old Russian leader, somewhat drastic in retrospect (4)
I believe the answer is:
tsar
'old russian leader' is the definition.
(I've seen this in another clue)
'somewhat drastic in retrospect' is the wordplay.
'somewhat' indicates a hidden word.
'in retrospect' says the letters should be written in reverse (in retrospect means looking back).
'rast' is hidden within 'drastic'.
'rast' back-to-front is 'TSAR'.
(Other definitions for tsar that I've seen before include "Alexander and Nicholas held this title in Russia" , "Absolute ruler" , "Monarch of old Russia" , "Russian leader" , "Powerful man" .)