The most common and most likely answer to this crossword clue is the 7 letter word KISSERS
I've seen this crossword clue in The Daily Telegraph
K
I
S
S
E
R
S
More clues leading to the result KISSERS
Buddy explains!

The straight part
We know that "to be seen under the mistletoe" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to be seen under the mistletoe = kissers.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 KISSERS mean?
KISSERS noun- One who kisses.
- Mouth.
- Face.
- The kissing gourami.
- A button on a bowstring that indicates consistent height when drawing a bow, for example by being placed against the archer's mouth .