Crossword Genius

Big brass twice contracted Yank HQ (5)

Ross

I believe the answer is:

tubas

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

'big brass' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for tubas that I've seen before include "What some musicians" , "Large brass instruments" .)

I've seen this clue in the Sydney Morning Herald.
Want a hint initially instead of a full solution? Install my app