The most common and most likely answer to this crossword clue is the 6 plus 6 letter word REGENT STREET
I've seen this crossword clue in The Daily Telegraph
R
E
G
E
N
T
S
T
R
E
E
T
Buddy explains!
The straight part
We know that "Powerful man having his way in London?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case powerful man having his way in london? = regent street.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.