Regularly insult crazy person (3)
I believe the answer is:
nut
'crazy person' is the definition.
(I've seen this before)
'regularly insult' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'insult' are 'NUT'.
(Other definitions for nut that I've seen before include "Bolt counterpart" , "Obsessive enthusiast" , "organized teachers" , "Bolt securer" , "piece of coal" .)