Guard by the southern gate (6)
I believe the answer is:
sentry
'guard' is the definition.
(sentry is a kind of guard)
'by the southern gate' is the wordplay.
'by' says to put letters next to each other.
'the southern' becomes 's' (abbreviation).
'gate' becomes 'entry' (I've seen this in another clue).
's'+'entry' is 'SENTRY'.
(Other definitions for sentry that I've seen before include "One keeps watch" , "Watchperson" , "One posted on guard" , "Military lookout" , "Soldier posted to keep watch" .)