Direct ambassador to a professor (4,2)
I believe the answer is:
head on
'direct' is the definition.
(similar in meaning)
'ambassador to a professor' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'professor' becomes 'don' (term for a university professor).
'he'+'a'+'don'='HEAD-ON'
(Other definitions for head on that I've seen before include "avoiding flinching" , "direct confrontation" , "without compromise" , "Sensibly keep this screwed" .)