Regularly daunt thin opponent (4)
I believe the answer is:
anti
'opponent' is the definition.
(someone opposed to something)
'regularly daunt thin' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'daunt'+'thin'='dauntthin'
The alternating letters of 'dauntthin' are 'ANTI'.
(Other definitions for anti that I've seen before include "Adversary" , "Not in favor of" , "on the other side" , "Someone opposed (to a policy?)" , "Prefix meaniing against" .)