Certificate before tea say, for ambassador (8)
I believe the answer is:
'for ambassador' is the definition.
(I've seen this before)
'certificate before tea' is the wordplay.
'certificate' becomes 'diploma' (diploma is a kind of certificate).
'before' says to put letters next to each other.
'tea' becomes 't' (I've seen this in other clues).
'say' is the link.
(Other definitions for diplomat that I've seen before include "One deals tactfully with others" , "National representative abroad" , "person with negotiating skills" , "Tactful type" , "Official representing a country abroad" .)