The most common and most likely answer to this crossword clue is the 10 letter word SHALLOWEST
I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "most superficial" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most superficial = shallowest.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 SHALLOWEST mean?
SHALLOWEST adjective- Having little depth; significantly less deep than wide.
- Extending not far downward.
- Concerned mainly with superficial matters.
- Lacking interest or substance.
- Not intellectually deep; not penetrating deeply; simple; not wise or knowing.
- Not deep in tone.
- Not far forward, close to the net.