Emperor seized by tyrant's army (4)
I believe the answer is:
tsar
'emperor' is the definition.
(I've seen this before)
'seized by tyrant's army' is the wordplay.
'seized by' says the answer is hidden in the clue (the answer has been seized by the clue).
'TSAR' can be found hidden inside 'tyrants army'.
(Other definitions for tsar that I've seen before include "Title of Russian emperor" , "Monarch of old Russia" , "Russian leader" , "Ruler finally executed" , "Nicholas II, say" .)