Prevent spoiling beard (5)
I believe the answer is:
debar
'prevent' is the definition.
(I know that debar is a more specific form of the action prevent)
'spoiling beard' is the wordplay.
'spoiling' indicates an anagram.
'beard' with letters rearranged gives 'DEBAR'.
(Other definitions for debar that I've seen before include "Black" , "Keep out" , "Block" , "Prevent from doing something" , "Stop" .)