The most common and most likely answer to this crossword clue is the 9 plus 5 letter word BUTTERFLY VALVE
I've seen this crossword clue in The Times
B
U
T
T
E
R
F
L
Y
V
A
L
V
E
Buddy explains!
The straight part
We know that "Regulator in carburettor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case regulator in carburettor = butterfly valve.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.