Tense anger about King’s ultimatum (6)
I believe the answer is:
threat
'ultimatum' is the definition.
(I've seen this in another clue)
'tense anger about king's' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'anger' becomes 'heat' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'king' becomes 'R' (abbreviation of Latin rex).
'heat' going around 'r' is 'hreat'.
't'+'hreat'='THREAT'
(Other definitions for threat that I've seen before include "Intimidatory words to exact compliance" , "Warning of intent to harm" , "A warning of harm to come" , "Statement of harmful intent" , "Impending danger" .)