The most common and most likely answer to this crossword clue is the 5 letter word ADULT
I've seen this crossword clue in Financial Times
A
D
U
L
T
More clues leading to the result ADULT
- Grown-up daughter wearing gold locket that's empty
- A mostly uninteresting time for grown-up
- Hand out lots now and again, but not for children
- See 3 Down
- Mature blue
- Responsible individual said, “every second counts”
- A fool drops love for university, getting mature
- A largely sombre time for a grown-up
- Figure that’s beyond a minority?
- Responsible person gives notice last month
Buddy explains!

The straight part
We know that "grown-up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grown-up = adult.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 ADULT mean?
ADULT noun- A fully grown human or animal.
- A person who has reached the legal age of majority.
- To (cause to) be or become an adult.
- To behave like an adult.
- Fully grown.
- Intended for or restricted to adults rather than children.
- Containing material of an explicit sexual nature; of, or pertaining to, pornography.
- Vulgar or profane.