Get everything ready and working then attack (3,4)
I believe the answer is:
set upon
'attack' is the definition.
(I know that set upon is a more specific form of the action attack)
'get everything ready and working' is the wordplay.
I cannot quite see how this works, but
'ready' could be 'setup' (synonyms) and 'setup' is located in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
A single letter 'o' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'then' acts as a link.
Can you help me to learn more?
(Other definitions for set upon that I've seen before include "Attacked" , "Determined to do (something)" , "Attack" , "Mug" .)