Performing plays regularly exclusively (4)
I believe the answer is:
only
'exclusively' is the definition.
('only' can be a synonym of 'exclusively')
'performing plays regularly' is the wordplay.
'performing' becomes 'on' (I've seen this in other clues).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'plays' are 'ly'.
'on'+'ly'='ONLY'
(Other definitions for only that I've seen before include "Exclusive - individual" , "Solely; except that" , "Single, solitary" , "Alone; solitary" , "Merely; single" .)