Slightly crazy about a joke (4)
I believe the answer is:
gaga
'slightly crazy' is the definition.
(I've seen this before)
'about a joke' is the wordplay.
'about' says the letters should be written in reverse.
'joke' becomes 'gag' (a gag is a joke or witticism).
'a'+'gag'='agag'
'agag' backwards is 'GAGA'.
(Other definitions for gaga that I've seen before include "dotty" , "bananas" , "Barmy joke, third in cracker" , "Slangily crazy or senile" , "US singer Lady ___" .)