Certificate before tea say, for ambassador (8)
I believe the answer is:
diplomat
'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).
'diploma'+'t'='DIPLOMAT'
'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" .)