The most common and most likely answer to this crossword clue is the 3 plus 3 plus 4 plus 2 plus 12 plus 6 letter word THE OLD LADY OF THREADNEEDLE STREET
I've seen this crossword clue in Independent
T
H
E
O
L
D
L
A
D
Y
O
F
T
H
R
E
A
D
N
E
E
D
L
E
S
T
R
E
E
T
Buddy explains!
The straight part
We know that "Funding source associated with London sewers?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case funding source associated with london sewers? = the old lady of threadneedle 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.