Stop cakes turning (4)
I believe the answer is:
snub
'stop' is the definition.
(snub can mean to stop or rebuff)
'cakes turning' is the wordplay.
'cakes' becomes 'buns' (cake is a type of bun).
'turning' says the letters should be written in reverse.
'buns' backwards is 'SNUB'.
(Other definitions for snub that I've seen before include "Rebuff; brush-off" , "Deliberate insult" , "Give (someone) the cold shoulder" , "Cold-shoulder" , "A rebuff for turned-up sort of nose" .)