Envoy bringing certificate on time (8)
I believe the answer is:
diplomat
'envoy' is the definition.
(I've seen this before)
'certificate on time' is the wordplay.
'certificate' becomes 'diploma' (diploma is a kind of certificate).
'on' says to put letters next to each other.
'time' becomes 't'.
'diploma'+'t'='DIPLOMAT'
'bringing' is the link.
(Other definitions for diplomat that I've seen before include "Official engaged in international negotiations" , "person with negotiating skills" , "Tactful person, may work in embassy" , "Official representing a country abroad" , "negotiator" .)