Greek character volunteers after article (5)
I believe the answer is:
theta
'greek character' is the definition.
(letter from the Greek alphabet)
'volunteers after article' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'after' says to put letters next to each other.
'article' becomes 'the' (the 'definite article' part of speech).
'ta' put after 'the' is 'THETA'.
(Other definitions for theta that I've seen before include "character in Marathon" , "Character from overseas" , "8th letter of the Greek alphabet" , "letter from abroad" , "Greek letter equivalent to th" .)