Act independently (2,2,5)
I think it could be either:
go it alone
'act independently' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for go it alone that I've seen before include "Act independently" , "set up as freelancer" , "Take something on, eschewing any help" , "Proceed without the help of others" , "act without help" .)