Old volunteer army producing letter from Greece (5)
I believe the answer is:
theta
'letter from greece' is the definition.
(letter from the Greek alphabet)
'old volunteer army' is the wordplay.
'old volunteer' becomes 'the' (I can't explain this - if you can you should believe this answer much more).
'army' becomes 'TA' (abbreviation for Territorial Army).
'the'+'ta'='THETA'
'producing' is the link.
Can you help me to learn more?
(Other definitions for theta that I've seen before include "Greek character" , "Letter from Greece" , "letter from abroad" , "Letter before iota" , "classic character?" .)