Cryptic crossword clue
Stunned, noting a mature reasoning 13

The most common and most likely answer to this crossword clue is the 13 letter word ARGUMENTATION

I've seen this crossword clue in The Daily Telegraph
A
R
G
U
M
E
N
T
A
T
I
O
N

Buddy explains!

The straight part
We know that "reasoning" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reasoning = argumentation.
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 ARGUMENTATION mean?

ARGUMENTATION noun
  1. Inference based on reasoning from given propositions.
  2. An exchange of arguments
  3. The addition of arguments to a model; parameterization.

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