Begin attack (3,5)
I believe the answer is:
set about
I believe this clue is a double definition.
'begin' is the first definition.
('set about' can be a synonym of 'begin')
'attack' is the second definition.
(thesaurus)
(Other definitions for set about that I've seen before include "Get cracking" , "Embark upon" , "Start to do something with determination" , "Begin - attack" .)