Crossword Genius

I didn't hear that engineer's announcement (4)

Ross

I believe the answer is:

what

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'i didn't hear' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for what that I've seen before include "Interrogatory word" , "Word used in a question" , "That which - come again!" , "I beg your pardon" , "Eh?" .)

I've seen this clue in The Independent.
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