What for Guatemalan flag's filling empty spot up (7)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

QUE (‘what’ in Spanish, for Guatemalan), then the central letters from (…’s filling) fLAg with ZiT (spot) minus the middle letter (empty) with all of that reversed (up) – &lit. This is the bird that appears (in an otherwise empty spot) on the Guatemalan flag, and also gives its name to the country’s currency. I could see how the clue worked, but did not know the necessary stuff, and plumped for ‘quetsal’, thinking ’empty spot’=’st’.