Letter from volunteers supporting article (5)
I believe the answer is:
theta
'letter' is the definition.
(theta is a kind of letter of the alphabet)
'volunteers supporting article' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'article' becomes 'the' (the 'definite article' part of speech).
'ta' put after 'the' is 'THETA'.
'from' acts as a link.
(Other definitions for theta that I've seen before include "8th letter of the Greek alphabet" , "classic character?" , "Character from overseas" , "character in Euripides" , "character in Marathon" .)