Emperor is some leader as tyrannical, in retrospect (4)
I believe the answer is:
tsar
'emperor' is the definition.
(I've seen this before)
'some leader as tyrannical in retrospect' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'in retrospect' says the letters should be written in reverse (in retrospect means looking back).
'rast' is hidden within 'leader as tyrannical'.
'rast' back-to-front is 'TSAR'.
'is' acts as a link.
(Other definitions for tsar that I've seen before include "Tyrant" , "Nicholas II was the last" , "Old Russian ruler" , "Rats (anag)" , "Old Russian 22" .)