Mask some of one's chagrin? (4)

I believe the answer is:

grin

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

'mask some' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'one's chagrin?' is the wordplay.
'GRIN' is hidden within the letters.
However, I am not sure how the hidden word is indicated.

'of' is the link.

Can you help me to learn more?

(Other definitions for grin that I've seen before include "Smirk" , "show ivories happily with it" , "Smile while pulling back the lips" , "Beam" , "Facial expression showing amusement or pleasure" .)

I've seen this clue in the Evening Standard.

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.