The most common and most likely answer to this crossword clue is the 6 letter word TENNER
I've seen this crossword clue in Financial Times
T
E
N
N
E
R
More clues leading to the result TENNER
Buddy explains!

The straight part
We know that "Cash" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cash = tenner.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 TENNER mean?
TENNER noun- A monetary note (bill) whose face value is ten basic units of currency. Originally, a ten-shilling (half pound) note.
- A kind of 10-kilowatt lamp.
- A tennis shoe.