Soldiers' dining room is a shambles (4)
I believe the answer is:
mess
I believe this is a double definition.
'soldiers' dining room' is the first definition.
(mess can mean a room where soldiers eat)
'a shambles' is the second definition.
(I've seen this in another clue)
'is' acts as a link.
(Other definitions for mess that I've seen before include "hash" , "Where sailors eat" , "dirty and unpleasant" , "Disagreeable mixture" , "Disorderly state" .)