Allied codebreaking project in the Second World War (5)

I believe the answer is:

ultra

(Other definitions for ultra that I've seen before include "Political extremist" , "holding extreme position" , "Person holding extreme views" , "Above, beyond, super" , "Secret intelligence" .)

I've seen this clue in The Times.

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.