Why the joke was unspoken? (3)

I believe the answer is:

gag

I'm a little stuck... Click here to teach me more about this clue!

'why' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for gag that I've seen before include "Funny" , "Silence the joke" , "Stop the mouth with a joke" , "Joke (colloq.)" , "that renders one speechless" .)

I've seen this clue in The Irish Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.