Dog's breakfast in canteen (4)
I believe the answer is:
mess
This clue is a double definition.
'dog's breakfast' is the first definition.
(I've seen this before)
'canteen' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for mess that I've seen before include "Where military personnel eat" , "Jumble" , "Chaos, confusion" , "Meal" , "dog's dinner" .)