Rehearse part of the play within cost (8)
I believe the answer is:
practice
'rehearse' is the definition.
('practice' can be a synonym of 'rehearse')
'part of the play within cost' is the wordplay.
'part of the play' becomes 'act' ('act' is part of 'play').
'within' is an insertion indicator.
'cost' becomes 'price' (price is a kind of cost).
'act' placed inside 'price' is 'PRACTICE'.
(Other definitions for practice that I've seen before include "Habit - procedure" , "''. . . . . . . . maked perfect''" , "Use" , "Doctor's premises" , "Doctor's business" .)