Part in a play; function (4)
I believe the answer is:
role
I believe this clue is a double definition.
'part in a play' is the first definition.
(I've seen this before)
'function' is the second definition.
(I know that function can be written as role)
(Other definitions for role that I've seen before include "Stage part" , "Part in play, film etc" , "Impersonation" , "A part of lore" , "Part played (in life?)" .)