Crossword Genius

Person holding cup? I bet that gets smashed — it’s what impatient waiters do (5,2,3,3)

Ross

I believe the answer is:

champ at the bit

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'person holding cup?' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'i bet that gets smashed it's what impatient waiters do' is the wordplay.
I cannot really see how this works, but
'i' could be 'a' and 'a' is present in the answer.
an anagram of 'that' is 'atth' which is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for champ at the bit that I've seen before include "Show impatience to begin" , "Be impatient to start" , "Be eager to begin" , "can't wait!" , "Be eager to start" .)

I've seen this clue in The Times.
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