The most common and most likely answer to this crossword clue is the 9 letter word INGENUOUS
I've seen this crossword clue in The Times
I
N
G
E
N
U
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S
More clues leading to the result INGENUOUS
- Naive or very clever, university expelling yours truly
- Frank is clever, dropping second of internationals for United
- Naive, but clever replacing one eye for you, say
- University for one clever and innocent
- Sincere and brilliant one expelled by university
- Naive Greek character defending dope trapped by debts
Buddy explains!

The straight part
We know that "Naive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case naive = ingenuous.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 INGENUOUS mean?
INGENUOUS adjective- Naive and trusting.
- Demonstrating childlike simplicity.
- Unsophisticated; clumsy or obvious.
- Unable to mask one's feelings.
- Straightforward, candid, open, and frank.