The most common and most likely answer to this crossword clue is the 5 letter word WRONG
I've seen this crossword clue in The Guardian
W
R
O
N
G
More clues leading to the result WRONG
- Grace maybe saving man — but there's wickedness!
- Disadvantage involving lover, ultimately, got girlfriend upset also
- Put at disadvantage immoral Labour leader
- Abuse was rained on new girl at first
- Grown apart, as not suitable
- Whatever is not just wide of the mark
- Grown out of Nick Park’s 1993 trousers?
- Not just a Chinese name entered by Rex
- Right opposite
- What rowing may become when ego is let loose!
Buddy explains!

The straight part
We know that "Not just/Not just" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not just/not just = wrong.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 WRONG mean?
WRONG noun- Something that is immoral or not good.
- An instance of wronging someone (sometimes with possessive to indicate the wrongdoer).
- The incorrect or unjust position or opinion.
- The opposite of right; the concept of badness.
- To treat unjustly; to injure or harm.
- To deprive of some right, or to withhold some act of justice.
- To slander; to impute evil to unjustly.
- Incorrect or untrue.
- Asserting something incorrect or untrue.
- Immoral, not good, bad.
- Improper; unfit; unsuitable.
- Not working; out of order.
- Designed to be worn or placed inward; as, the wrong side of a garment or of a piece of cloth.
- Twisted; wry.
- In a way that isn't right; incorrectly, wrongly.