The most common and most likely answer to this crossword clue is the 6 plus 4 letter word CREDIT SLIP
I've seen this crossword clue in The Guardian
C
R
E
D
I
T
S
L
I
P
Buddy explains!
The straight part
We know that "purchased with this?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case purchased with this? = credit slip.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.