Male entering race ahead of a Greek character (5)
I believe the answer is:
theta
'greek character' is the definition.
(letter from the Greek alphabet)
'male entering race ahead of a' is the wordplay.
'male' becomes 'he'.
'entering' is an insertion indicator.
'race' becomes 'tt'.
'ahead of' says to put letters next to each other (I've seen this in other clues).
'he' going inside 'tt' is 'thet'.
'thet'+'a'='THETA'
(Other definitions for theta that I've seen before include "Letter before iota" , "letter to Greeks" , "letter from Argos?" , "Homeric character" , "character from Greece" .)