United with fool (3)
I believe the answer is:
nit
'fool' is the definition.
(both can mean a stupid person)
'united with' is the wordplay.
'with' indicates a hidden word.
'NIT' is hidden in the letters of 'united'.
(Other definitions for nit that I've seen before include "no egghead?" , "Egg; fool" , "Hair louse" , "Foolish person" , "Louse's egg - berk" .)