Greek character telephones, initially, before he volunteers (5)
I believe the answer is:
theta
'greek character' is the definition.
(letter from the Greek alphabet)
'telephones initially before he volunteers' is the wordplay.
'initially' indicates taking the first letters.
'before' says to put letters next to each other.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
The first letter of 'telephones' is 't'.
't'+'he'+'ta'='THETA'
(Other definitions for theta that I've seen before include "letter from abroad" , "character in Euripides" , "Letter to the Corinthians" , "Letter before iota" , "Greek letter equivalent to th" .)