Work in regressive places for bung (4,2)
I believe the answer is:
stop up
'bung' is the definition.
'work in regressive places' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'in' means one lot of letters goes inside another.
'regressive' says the letters should be written in reverse.
'places' becomes 'puts' ('put' can be a synonym of 'place').
'puts' backwards is 'stup'.
'op' placed into 'stup' is 'STOP UP'.
'for' acts as a link.
(Other definitions for stop up that I've seen before include "close" , "Don't retire here" , "Don't go to bed early" , "Cork" , "Block (eg an earth)" .)