Belief is conveyed by ultimatum, not half (5)
I believe the answer is:
deism
'belief' is the definition.
(deism is a kind of belief)
'is conveyed by ultimatum not half' is the wordplay.
I cannot quite see how this works, but
'is' is present in the answer.
The remaining letters 'dem' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for deism that I've seen before include "Belief in the existence of God" , "Belief in a god from reason not revelation" , "Belief in a supreme being who does not intervene in the universe" , "Belief in a creator" .)