Be ominous for Emperor (not half) and King (5)
I believe the answer is:
augur
'be ominous for' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'emperor not half and king' is the wordplay.
'emperor' becomes 'aug' (I can't explain this - if you can you should believe this answer much more).
'not' becomes 'un' (I've seen this in other clues).
'half' suggests halving the letters.
'and' says to put letters next to each other.
'king' becomes 'r' (r is abbreviation for rex).
'un' with half the letters taken is 'u'.
'aug'+'u'+'r'='AUGUR'
Can you help me to learn more?
(Other definitions for augur that I've seen before include "Portend a good or/bad outcome" , "Portend; Roman official" , "Predict as from omens" , "Presage, portend" , "Foretell, be a sign of" .)