Saucy American oath arising in vacuous country (8)

'saucy' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'american oath arising in vacuous country' is the wordplay.
'american' becomes 'US' (United States).
'oath' becomes 'drat' (I can't explain this - if you can you should believe this answer much more).
'arising' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'vacuous' means to remove the middle letters.
'country' with its centre removed is 'cy'.
'drat' backwards is 'tard'.
'us'+'tard'='ustard'
'ustard' going inside 'cy' is 'CUSTARDY'.

Can you help me to learn more?