Cracked back "Will flabbergast you" (4)
I believe the answer is:
stun
'flabbergast you' is the definition.
'stun' can be an answer for 'flabbergast' (I've seen this in another clue). I am not certain of the 'you' bit.
'cracked back' is the wordplay.
'cracked' becomes 'nuts' (synonyms).
'back' is a reversal indicator.
'nuts' backwards is 'STUN'.
'will' acts as a link.
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" .)