The most common and most likely answer to this crossword clue is the 5 letter word RIPEN
I've seen this crossword clue in The Guardian
R
I
P
E
N
More clues leading to the result RIPEN
- Become ready to eat beef — no good before noon
- Mature writer on middle of trip
- Mature with no time for nonsense? Essentially responsible!
- … iron, not working over fold, to turn red, perhaps?
- Become ready to eat carp — no good before noon
- Mature writer, following religious instruction
- Mature writing with Apple device?
- Mature beef with no starter and drop of Nolly?
- Mature writer on Christian discipline?
- Wiser in the end, I turn out mature
Buddy explains!

The straight part
We know that "Mature" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mature = ripen.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 RIPEN mean?
RIPEN verb- To grow ripe; to become mature (said of grain, fruit, flowers etc.)
- To approach or come to perfection.
- To cause to mature; to make ripe
- To mature; to fit or prepare; to bring to perfection