Suppress a joke (3)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'suppress' is the first definition.
'a joke' is the second definition.
('gag' can be a synonym of 'joke')
(Other definitions for gag that I've seen before include "To silence", "Joke; restriction", "Palindromic muzzle", "Silence the joke", "Muzzle in palindrome form", "A joke to muffle?", "Joke; silencer", "Muffle with slang joke", "A palindromic joke or muzzle" and "Stop the mouth with a joke".)