Not a stage whisper (unless you have crazy ideas) (5)
I believe the answer is:
aside
'not a stage whisper unless you' is the definition.
I can't judge whether this defines the answer.
'crazy ideas' is the wordplay.
'crazy' indicates anagramming the letters.
'ideas' with letters rearranged gives 'ASIDE'.
'have' is the link.
Can you help me to learn more?
(Other definitions for aside that I've seen before include "Stage whisper" , "Out of other people's hearing" , "Remark not for hearing" , "Whispered comment" , "Separately" .)