Work in regressive places for bung (4,2)
I believe the answer is:
Here is my best explanation:
'bung' is the definition.
'work in regressive places for' is the subsidiary indication.
'work' becomes 'op' (abbreviation for opus).
'in' means one lot of letters goes inside another.
'regressive' says the letters should be written backwards.
'places for' becomes 'puts' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'puts' backwards is 'stup'.
'op' placed into 'stup' is 'stopup'.
(Other definitions for stop up that I've seen before include "close", "Don't retire here", "Don't go to bed early", "Cork" and "Block (eg an earth)".)