Certificate ambassador detailed (7)
I believe the answer is:
diploma
'certificate' is the definition.
(diploma is a kind of certificate)
'ambassador detailed' is the wordplay.
'ambassador' becomes 'diplomat' (I've seen this before).
'detailed' means to remove the last letter (the word must be end-less or 'de-tailed').
'diplomat' with its final letter taken off is 'DIPLOMA'.
(Other definitions for diploma that I've seen before include "Document conferring a qualification" , "documentation" , "Certificate of education" , "Honorific document" , "Course certificate" .)