Ivan the Terrible is conceited (4)
I believe the answer is:
vain
'is conceited' is the definition.
(synonyms)
'ivan the terrible' is the wordplay.
'the terrible' indicates anagramming the letters.
'ivan' anagrammed gives 'VAIN'.
(Other definitions for vain that I've seen before include "Conceited and ineffectual" , "thoughtless" , "Egotistical" , "being a big-head" , "Useless; conceited" .)