One by one, as part of the act (2,4)
I believe the answer is:
in turn
'one by one' is the definition.
('in turn' means one by one)
'part of the act' is the wordplay.
'part of' becomes 'in' (I've seen this in other clues).
'the act' becomes 'turn' (turn is a kind of act**).
'in'+'turn'='IN TURN'
'as' is the link.
(Other definitions for in turn that I've seen before include "One after another" , "not out of place" , "orderly treatment" , "a succession of things happen" , "in an orderly fashion" .)