Crazy as singing lady a joke upset (4)
I believe the answer is:
gaga
'crazy' is the definition.
(I know that crazy can be written as gaga)
'a joke upset' is the wordplay.
'joke' becomes 'gag' (a gag is a joke or witticism).
'upset' says the letters should be written in reverse.
'a'+'gag'='agag'
'agag' reversed gives 'GAGA'.
'as singing lady' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for gaga that I've seen before include "Slightly mad (colloq.)" , "Slangily crazy or senile" , "Lady who sings" , "Barmy joke, third in cracker" , "Queen's radio recording" .)