The most common and most likely answer to this crossword clue is the 8 letter word RELIANCE
I've seen this crossword clue in The Sunday Telegraph
R
E
L
I
A
N
C
E
Buddy explains!

The straight part
We know that "Banking" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case banking = reliance.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 RELIANCE mean?
RELIANCE noun- The act of relying (on or in someone or something); trust.
- The condition of being reliant or dependent.
- Anything on which to rely; ground of trust.
- A person or thing which relies on another.