... thus King's mortified (4)
I believe the answer is:
sick
'mortified' is the definition.
(both can mean disappointed or despondent)
'thus king's' is the wordplay.
'thus' becomes 'sic' (synonyms).
'king' becomes 'k' (abbreviation for king in chess).
'sic'+'k'='SICK'
(Other definitions for sick that I've seen before include "Ill, unwell" , "Ill; nauseous" , "not good" , "vomit" , "Bad" .)