The most common and most likely answer to this crossword clue is the 4 plus 7 letter word MARY POPPINS
I've seen this crossword clue in The Guardian
M
A
R
Y
P
O
P
P
I
N
S
Buddy explains!
The straight part
We know that "Banks employed her" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case banks employed her = mary poppins.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.