Prevent bread going bad (5)
I believe the answer is:
debar
'prevent' is the definition.
(I know that debar is a more specific form of the action prevent)
'bread going bad' is the wordplay.
'going bad' indicates an anagram.
'bread' with letters rearranged gives 'DEBAR'.
(Other definitions for debar that I've seen before include "Shut out, preclude" , "Prohibit (from doing something)" , "Exclude from a place" , "Prevent from doing something" , "Exclude (from doing something)" .)