Soldiers moving south for dope (3)
I believe the answer is:
ass
'dope' is the definition.
(both can mean a stupid person)
'soldiers moving south' is the wordplay.
'soldiers moving' becomes 'as' (I am not sure about this - if you are sure you should believe this answer much more).
'south' becomes 'S' (abbreviation).
'as'+'s'='ASS'
'for' is the link.
(Other definitions for ass that I've seen before include "Silly fool or beast of burden" , "Charlie" , "Small hoofed animal" , "Horse relative" , "Turkey" .)