Fool is half in agreement (3)
I believe the answer is:
ass
'fool' is the definition.
(ass is a kind of fool)
'is half in agreement' is the wordplay.
'half' means to take half the letters.
'in' means one lot of letters goes inside another.
'agreement' becomes 'as' (I can't explain this - if you can you should give a lot more credence to this answer).
'is' with half the letters taken is 's'.
's' put inside 'as' is 'ASS'.
Can you help me to learn more?
(Other definitions for ass that I've seen before include "Titania loved one" , "Horse hybrid" , "Pompous person" , "Horse relative" , "Foolish person; animal" .)