Understudy's study (4)
I believe the answer is:
role
'understudy's study' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for role that I've seen before include "Impersonation" , "Acting part" , "Part played (in life?)" , "Part, portrayal" , "Part in play, film etc" .)