Part of main dynamics for form of motor racing (4)
I believe the answer is:
indy
'form of motor racing' is the definition.
(I've seen this before)
'part of main dynamics for' is the wordplay.
'part of' says the answer is hidden in the clue.
'INDY' is hidden in the letters of 'main dynamics for'.
(Other definitions for indy that I've seen before include "US form of motor racing" , "- - Car racing (in US)" , "US car race" , "Form of competition between drivers" , "US motor-racing" .)