Prohibition involving shout that startles monkey (6)
I believe the answer is:
baboon
'monkey' is the definition.
(baboon is a kind of monkey)
'prohibition involving shout that startles' is the wordplay.
'prohibition' becomes 'ban' (synonyms).
'involving' indicates putting letters inside.
'shout that startles' becomes 'boo' (noise made to scare someone).
'ban' enclosing 'boo' is 'BABOON'.
(Other definitions for baboon that I've seen before include "A large one of 17 across" , "Large monkey with a doglike muzzle" , "Stupid person" , "Type of large monkey" , "Large primate" .)