Public official almost set up shop (3,2)
I believe the answer is:
rat on
'shop' is the definition.
(both can mean to betray someone)
'public official almost set up' is the wordplay.
'public official' becomes 'notary' (I've seen this before).
'almost' means to remove the last letter (almost the whole word is kept).
'set up' is a reversal indicator (in a down clue, letters go up).
'notary' with its last letter removed is 'notar'.
'notar' reversed gives 'RAT ON'.
(Other definitions for rat on that I've seen before include "desert" , "Betray" , "shop" , "Give away" , "grass up" .)