Warships advance South (4)
I believe the answer is:
subs
'warships' is the definition.
(sub is a kind of warship)
'advance south' is the wordplay.
'advance' becomes 'sub' (I've seen this in other clues**).
'south' becomes 's' ('S' can be a synonym of 'south').
'sub'+'s'='SUBS'
(Other definitions for subs that I've seen before include "Reserves at football (sl.)" , "Club members' payments" , "Players in reserve" , "Advance payments" , "vehicles" .)