Frustrated in fencing? (6)

I believe the answer is:

foiled

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

'frustrated' is the definition.
('foil' can be a synonym of 'frustrate')

I don't understand the remainder of the clue.

(Other definitions for foiled that I've seen before include "Thwarted, defeated" , "Thwarted, frustrated" , "Thwarted with sword?" , "Prevented the success of" .)

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.