The most common and most likely answer to this crossword clue is the 8 plus 3 letter word CHESHIRE CAT
I've seen this crossword clue in Financial Times
C
H
E
S
H
I
R
E
C
A
T
More clues leading to the result CHESHIRE CAT
Buddy explains!
The straight part
We know that "One can’t stop smiling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one can’t stop smiling = cheshire cat.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.