Regularly giggle at his imperfect form and cause him to doubt himself (8)
I believe the answer is:
gaslight
'form and cause him to doubt himself' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'regularly giggle at his imperfect' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'imperfect' indicates an anagram.
The alternate letters of 'giggle' are 'ggl'.
'ggl'+'at'+'his'='gglathis'
'gglathis' is an anagram of 'GASLIGHT'.
Can you help me to learn more?
(Other definitions for gaslight that I've seen before include "Old film" , "Old form of illumination" , "Sort of illumination" , "source of illumination" , "Old-fashioned illumination" .)