A play part or play a part (3)
I believe the answer is:
act
This clue is a double definition.
'a 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)
'or' acts as a link.
(Other definitions for act that I've seen before include "Major section of a play" , "Feat" , "Do something, in a play?" , "Major division of play" , "Perform - part of a play" .)