Perhaps first secretary's graduation certificate, first of three (8)
I believe the answer is:
diplomat
'perhaps first secretary's' is the definition.
(first secretary is a type of diplomat)
'graduation certificate first of three' is the wordplay.
'graduation certificate' becomes 'diploma' (a diploma is given when graduating).
'first of three' becomes 't' (1st letter of 'three').
'diploma'+'t'='DIPLOMAT'
(Other definitions for diplomat that I've seen before include "negotiator" , "Our Man in Havana" , "An ambassador or tactful person" , "Tactful type" , "National representative abroad" .)