This is a deletion - look for an indicator that suggests removing or taking away certain letters from a word.
The phrase '(not American)' indicates you should remove 'US' from 'Aussie'. After removing those letters, you need to interpret what remains: 'A' becomes 'ONE' and you're left with 'SIE'.
Answer: ONESIE | Remove 'US' (American) from 'Aussie' to get 'A' + 'SIE'. Then 'A' = 'ONE', giving you ONE + SIE = ONESIE (nightwear).