The most common and most likely answer to this crossword clue is the 5 letter word RIPER
R
I
P
E
R
Buddy explains!

The straight part
We know that "More mature" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case more mature = riper.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 RIPER mean?
RIPER adjective- (of fruits, vegetables, seeds etc.) Ready for reaping or gathering; having attained perfection; mature
- (of foods) Advanced to the state of fitness for use; mellow
- Having attained its full development; mature; perfected
- Maturated or suppurated; ready to discharge; -- said of sores, tumors, etc.
- Ready for action or effect; prepared.
- Like ripened fruit in ruddiness and plumpness.
- Intoxicated.
- Of a conflict between parties, having developed to a stage where the conflict may be reviewed by a court of law.
- Smelly: having a disagreeable odor.