Dry answer about ambassador's character (5)
I believe the answer is:
theta
'character' is the definition.
(letter of the Greek alphabet)
'dry answer about ambassador's' is the wordplay.
'dry' becomes 'tt' (abbreviation for teetotal).
'answer' becomes 'a' (abbreviation eg in Q and A).
'about' is an insertion indicator.
'ambassador' becomes 'HE' (His or Her Excellency).
'tt'+'a'='tta'
'tta' enclosing 'he' is 'THETA'.
(Other definitions for theta that I've seen before include "character in Marathon" , "Eighth letter of Greek alphabet" , "classic character?" , "Character from overseas" , "character in Euripides" .)