You and I organise seal on traitor (6)
I believe the answer is:
weasel
'traitor' is the definition.
(weasel can mean a treacherous person)
'you and i organise seal on' is the wordplay.
'you and i' becomes 'we' ().
'organise' indicates anagramming the letters.
'on' is an insertion indicator (as in clothing 'on' a person).
'seal' anagrammed gives 'asel'.
'we' inserted inside 'asel' is 'WEASEL'.
(Other definitions for weasel that I've seen before include "Sly type" , "This small mammal saw eel" , "Boggle" , "Small fierce animal with slender body and reddish fur" , "These sort of words are meaningless" .)