Theory regularly intervenes in crowd controls (7)
I believe the answer is:
masters
'controls' is the definition.
(I know this)
'theory regularly intervenes in crowd' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'intervenes in' means one lot of letters goes inside another.
'crowd' becomes 'mass' (massing is a kind of crowding).
The alternate letters of 'theory' are 'ter'.
'ter' inserted inside 'mass' is 'MASTERS'.
(Other definitions for masters that I've seen before include "Higher degree" , "Male teachers" , "competition today" , "Boys" , "Could be checkers" .)