The most common and most likely answer to this crossword clue is the 5 plus 4 letter word UPPER DECK
I've seen this crossword clue in Financial Times
U
P
P
E
R
D
E
C
K
Buddy explains!
The straight part
We know that "Part of London bus/overlooked from the bridge" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of london bus/overlooked from the bridge = upper deck.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.