Cryptic crossword clue
Mounting kind of bubbly old horse, a source of speed 12

The most common and most likely answer to this crossword clue is the 12 letter word TURBOCHARGER

I've seen this crossword clue in The Times
T
U
R
B
O
C
H
A
R
G
E
R

Buddy explains!

The straight part
We know that "a source of speed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a source of speed = turbocharger.
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 TURBOCHARGER mean?

TURBOCHARGER noun
  1. Inlet air compressor for an internal combustion engine (either Otto or Diesel cycle), powered from the exhaust air.

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