The most common and most likely answer to this crossword clue is the 7 plus 6 letter word TRAFFIC SIGNAL
I've seen this crossword clue in The Times
T
R
A
F
F
I
C
S
I
G
N
A
L
Buddy explains!
The straight part
We know that "Means of controlling the flow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case means of controlling the flow = traffic signal.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.