Be disloyal to (6)
I believe the answer is:
betray
'be disloyal to' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for betray that I've seen before include "Sell down the river" , "Hand someone over to an enemy" , "Give away" , "Inadvertently reveal" , "Inform on (someone)" .)