The most common and most likely answer to this crossword clue is the 8 letter word FLYWHEEL
I've seen this crossword clue in The Daily Telegraph
F
L
Y
W
H
E
E
L
Buddy explains!

The straight part
We know that "Machine regulator" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case machine regulator = flywheel.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 FLYWHEEL mean?
FLYWHEEL noun- A rotating mass used to maintain the speed of a machine within certain limits while the machine receives or releases energy at a varying rate.