Wow with crazy somersaulting (4)
I believe the answer is:
stun
'wow' is the definition.
(wow can mean to stun or astonish)
'crazy somersaulting' is the wordplay.
'crazy' becomes 'nuts' (nuts can informally mean crazy or mad).
'somersaulting' says the letters should be written in reverse (letters flip or somersault over).
'nuts' backwards is 'STUN'.
'with' is the link.
(Other definitions for stun that I've seen before include "Nuts (anag)" , "Surprise greatly or daze" , "Render temporarily senseless" , "Daze, render semi-conscious" , "Numb with nuts" .)