The most common and most likely answer to this crossword clue is the 11 letter word SPEEDOMETER
I've seen this crossword clue in The Times
S
P
E
E
D
O
M
E
T
E
R
Buddy explains!

The straight part
We know that "gauge for HS2" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gauge for hs2 = speedometer.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 SPEEDOMETER mean?
SPEEDOMETER noun- A device that measures, and indicates the current speed of a vehicle.
- Such a device incorporating an odometer.