Play part, or play a part (3)
I believe the answer is:
act
I believe this is a double definition.
'play part' is the first definition.
('act' is part of 'play')
'play a part' is the second definition.
(I've seen this in another clue)
(Other definitions for act that I've seen before include "A deed or statute" , "Start of a drama" , "work" , "turn" , "Routine" .)