A Greek character joins southern alliance (4)
I believe the answer is:
axis
'alliance' is the definition.
(alliance between two countries)
'a greek character joins southern' is the wordplay.
'greek character' becomes 'xi' (letter from the Greek alphabet**).
'joins' is a charade indicator (letters next to each other).
'southern' becomes 's' (abbreviation).
'a'+'xi'+'s'='AXIS'
(Other definitions for axis that I've seen before include "Imaginary centre of rotation" , "Spotted deer" , "(Rotational?) alliance of powers" , "Line through the middle" , "league" .)