Betray not a king - it's retrograde (3,2)
I believe the answer is:
rat on
'betray' is the definition.
('rat on' can be a synonym of 'betray')
'not a king it's retrograde' is the wordplay.
'a king' becomes 'ar' (I've seen this in other clues**).
'it's retrograde' is a reversal indicator.
'not'+'ar'='notar'
'notar' back-to-front is 'RAT ON'.
(Other definitions for rat on that I've seen before include "Betray" , "shop" , "desert" , "grass up" , "Give away" .)