The most common and most likely answer to this crossword clue is the 5 letter word TOOTS
I've seen this crossword clue in The Guardian
T
O
O
T
S
Buddy explains!

The straight part
We know that "US sweetheart/whistles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case us sweetheart/whistles = toots.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TOOTS mean?
TOOTS noun- The noise of a horn or whistle.
- (by extension) A fart; flatus.
- Cocaine.
- A portion of cocaine that a person snorts.
- A spree of drunkness.
- (pronounced /tʊt/) Rubbish; tat.
- A message on the social networking software Mastodon.
- To stand out, or be prominent.
- To peep; to look narrowly.
- To see; to spy.
- To flatulate.
- To make the sound of a horn or whistle.
- To cause a horn or whistle to make its sound.
- To go on a drinking binge.
- To snort (a recreational drug).
- To post a message on a Mastodon instance (a self-hosted version of the networking software).
- A toilet.
- (sometimes derogatory) Babe, sweetie: a term used when addressing a young woman, besties, sisters.
- A gender-neutral term of affection.