Attack arrangement that's working (3,4)
I believe the answer is:
set upon
'attack' is the definition.
(I know that set upon is a more specific form of attack)
'arrangement that's working' is the wordplay.
'arrangement' becomes 'set up' (I've seen this in another clue).
'that's working' becomes 'on' (e.g. a machine is working or on).
'setup'+'on'='SET UPON'
(Other definitions for set upon that I've seen before include "Mug" , "Attack" , "Determined to do (something)" , "Attacked" .)