Play a part (3)
I believe the answer is:
act
'play a part' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for act that I've seen before include "Major section of a play" , "Do something, in a play?" , "Perform theatrically" , "Do something (on stage?)" , "Behave like thespian" .)