Pickle or jam in army dining room (4)
I believe the answer is:
mess
I believe this is a triple definition!
'pickle' is the first definition.
(both can mean a difficult situation)
'or' is a link.
'jam' is the second definition.
(both can mean a difficult situation)
'in' is a link.
'army dining room' is the third definition.
(mess can mean a room for eating food)
(Other definitions for mess that I've seen before include "Service dining-area" , "Untidy muddle" , "Where those who serve eat" , "pickle" , "Army dining place" .)