The most common and most likely answer to this crossword clue is the 5 plus 5 letter word PAPER MONEY
I've seen this crossword clue in The Guardian
P
A
P
E
R
M
O
N
E
Y
More clues leading to the result PAPER MONEY
Buddy explains!
The straight part
We know that "Cash now only in £50 notes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cash now only in £50 notes = paper money.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.