Imbecile soldiers moving south (3)
I believe the answer is:
ass
'imbecile' is the definition.
(thesaurus)
'soldiers moving south' is the wordplay.
'soldiers moving' becomes 'as' (I can't explain this - if you can you should give a lot more credence to this answer).
'south' becomes 'S' (synonyms).
'as'+'s'='ASS'
(Other definitions for ass that I've seen before include "Wally" , "Donkey - fool" , "jack" , "Animal related to horses" , "Horse hybrid" .)