Cryptic crossword clue
Tail of aircraft worked very nicely, shortening by 50% around Mach 1 9

The most common and most likely answer to this crossword clue is the 9 letter word TRANSONIC

I've seen this crossword clue in The Times
T
R
A
N
S
O
N
I
C

Buddy explains!

The straight part
We know that "around Mach 1" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case around mach 1 = transonic.
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 TRANSONIC mean?

TRANSONIC adjective
  1. Just below, or just above the speed of sound (0.8 < Ma < 1.2 approximately).
  2. Passing from subsonic to supersonic, or vice versa.

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