Ambassador in temperate area — a character! (5)
I believe the answer is:
theta
'a character' is the definition.
(letter of the Greek alphabet)
'ambassador in temperate area' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'in' indicates putting letters inside.
'temperate' becomes 'TT' (short for teetotal).
'area' becomes 'a' (maths abbreviation).
'he' going within 'tt' is 'thet'.
'thet'+'a'='THETA'
(Other definitions for theta that I've seen before include "letter from abroad" , "Foreign letter" , "character in Marathon" , "letter to Greeks" , "Letter before iota" .)