Article on old army character from abroad (5)
I believe the answer is:
theta
'character from abroad' is the definition.
(letter of the Greek alphabet)
'article on old army' is the wordplay.
'article' becomes 'the' (the definite article).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'old army' becomes 'TA' (former name for the UK Army Reserve).
'the'+'ta'='THETA'
(Other definitions for theta that I've seen before include "Letter from Greece" , "Homeric character" , "Greek character" , "Greek letter used in trigonometry" , "Foreign letter" .)