Military cafeteria (4)
I believe the answer is:
'military cafeteria' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mess that I've seen before include "sloppy > food" , "dirty and unpleasant" , "Where military personnel eat" , "Officers' restaurant - what a dump" , "State of confusion or untidiness" .)