Adversary fooled regularly (3)
I believe the answer is:
foe
'adversary' is the definition.
(I know that foe is a type of adversary)
'fooled regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'fooled' are 'FOE'.
(Other definitions for foe that I've seen before include "opposing side" , "Evil rival" , "Opposition" , "Opponent" , "Antagonist" .)