Playing in position as substitute (6)
I believe the answer is:
acting
I believe this is a double definition.
'playing' is the first definition.
('act' can be a synonym of 'play')
'in position as substitute' is the second definition.
(e.g. acting president)
(Other definitions for acting that I've seen before include "Interim" , "Temporary - work in the theatre" , "putting it on" , "in a provisional role" , "Caretaker" .)