The most common and most likely answer to this crossword clue is the 8 letter word RIPENING
I've seen this crossword clue in The Times
R
I
P
E
N
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G
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The straight part
We know that "maturing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case maturing = ripening.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 RIPENING mean?
RIPENING 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
- The process of becoming ripe.