Crossword Genius

Sound made by tuba lacking a lot of energy? (5)

Ross

I believe the answer is:

oomph

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'energy?' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for oomph that I've seen before include "Informal energy or enthusiasm, usually in a girl" , "Go" , "Vitality, zing" , "drive" , "Enthusiasm - vigour - sex appeal" .)

I've seen this clue in The Daily Mail.
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