Cracked back "Will flabbergast you" (4)
I believe the answer is:
'will flabbergast you' is the definition.
I can't judge whether this defines the answer.
'cracked back' is the wordplay.
'cracked' becomes 'nuts' (synonyms).
'back' shows that the letters should be reversed in order.
'nuts' reversed gives 'STUN'.
Can you help me to learn more?
(Other definitions for stun that I've seen before include "Confound" , "Dumbfound" , "Surprise greatly or daze" , "Daze with nuts" , "Render insensitive" .)