Right to enter embassy following promotion (9)
I believe the answer is:
admission
'right to enter' is the definition.
(I've seen this before)
'embassy following promotion' is the wordplay.
'embassy' becomes 'mission' (I've seen this before).
'following' means one lot of letters go next to another.
'promotion' becomes 'ad' (ad is a kind of promotion).
'mission' after 'ad' is 'ADMISSION'.
(Other definitions for admission that I've seen before include "yield confession of sin maybe" , "Ticket holders expect this" , "Entrance fee" , "Anything conceded" , "Right to go in" .)