The most common and most likely answer to this crossword clue is the 7 letter word CUPCAKE
I've seen this crossword clue in The Daily Telegraph
C
U
P
C
A
K
E
Buddy explains!

The straight part
We know that "Sweetheart in America" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sweetheart in america = cupcake.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 CUPCAKE mean?
CUPCAKE noun- A small cake baked in a paper container shaped like a cup, often with icing on top.
- An attractive young woman.
- A weak or effeminate man.
- Term of endearment
- To flirt; to talk or act amorously or intimately.