Emperor in knight's armour (4)
I believe the answer is:
tsar
'emperor' is the definition.
(I've seen this before)
'in knight's armour' is the wordplay.
'in' says the answer is hidden in the clue.
'TSAR' is hidden in the letters of 'knights armour'.
(Other definitions for tsar that I've seen before include "Star for a Russian emperor" , "Monarch of old Russia" , "Alexander and Nicholas held this title in Russia" , "Word derived from Caesar, used by European rulers since 1547" , "Tyrant" .)