Best said, 'Oh you ... idiot!' (6)
I believe the answer is:
outwit
'best' is the definition.
(thesaurus)
'said oh you idiot' is the wordplay.
'said' shows a homophone (sound like).
'you' becomes 'u' (I've seen this in other clues).
'idiot' becomes 'twit' (I've seen this in other clues).
'oh' is a homophone of 'o'.
'o'+'u'+'twit'='OUTWIT'
(Other definitions for outwit that I've seen before include "Outsmart" , "Intellectually get the better of" , "Surpass in ingenuity" , "Mentally get the better of" , "Outfox" .)