Gymnastic feat where champion hit the roof (8)
I believe the answer is:
backflip
'gymnastic feat' is the definition.
(type of somersault)
'champion hit the roof' is the wordplay.
'champion' becomes 'back' (I've seen this in other clues).
'hit the roof' becomes 'flip' (both can mean to become very angry).
'back'+'flip'='BACKFLIP'
'where' acts as a link.
(Other definitions for backflip that I've seen before include "Type of somersault in gymnastics" , "Reverse somersault" .)