The most common and most likely answer to this crossword clue is the 6 letter word MATURE
I've seen this crossword clue in Financial Times
M
A
T
U
R
E
More clues leading to the result MATURE
- Fully developed type demanding change of leader
- Fully-developed horse bearing 5? Not half!
- When texting, you are received by friend that's grown-up
- Adult companion grabbing usher’s outside
- In essence first character goes back to the previous season
- Victor behaving like an adult?
- Spiking tea with rum? Grow up!
- Adult scholar with true disposition
- Victor, old player from United, runs into friend
- Victor was no spring chicken
Buddy explains!

The straight part
We know that "Victor/behaving like an adult" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case victor/behaving like an adult = mature.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 MATURE mean?
MATURE adjective- Fully developed; grown up in terms of physical appearance, behaviour or thinking; ripe.
- Brought to a state of complete readiness.
- Profound; careful.
- Come to, or in a state of, completed suppuration.
- Suitable for adults only, due to sexual themes, violence, etc.
- To proceed toward maturity: full development or completion (either of concrete or of abstract things, e.g. plans, judgments, qualities).
- (of food, especially fruit) To attain maturity, to become mature or ripe.
- To bring (something) to maturity, full development or completion.
- To make (something, e.g. fruit) ripe or mature.
- (of a person) To proceed toward or become mature or full-grown, either physically or psychologically; to gain experience or wisdom with age.
- To make (someone) mature.
- To reach the date when payment is due.