In Subotica, very deep chamber (4)
I believe the answer is:
cave
'chamber' is the definition.
(I have seen 'Underground chamber' mean 'cave' so perhaps 'chamber' could also mean 'cave')
'in subotica very deep' is the wordplay.
'in' indicates the answer is hidden within the clue.
'CAVE' can be found hidden inside 'subotica very deep'.
(Other definitions for cave that I've seen before include "Large grotto" , "Beware" , "Underground enclosure with surface access" , "Hollow in a rock" , "Hollow - warning" .)