One by one forming 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'
'forming' is the link.
(Other definitions for in turn that I've seen before include "a succession of things happen" , "One after another" , "orderly treatment" , "not out of place" , "in an orderly fashion" .)