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

I believe the answer is:

what

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

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

I don't understand the remainder of the clue.

(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.

Want a hint initially instead of a full solution? Install my app