Bag for instance (4)
I believe the answer is:
case
This clue is a double definition.
'bag' is the first definition.
(I have seen 'Overnight bag' mean 'case' so perhaps 'bag' could also mean 'case')
'instance' is the second definition.
('case' can be a synonym of 'instance')
'for' acts as a link.
(Other definitions for case that I've seen before include "Box of twelve bottles of wine" , "Patient - luggage" , "Patient; bag" , "Check out" , "Container and court proceeding" .)