The most common and most likely answer to this crossword clue is the 5 letter word PAYER
I've seen this crossword clue in The Daily Telegraph
P
A
Y
E
R
Buddy explains!
The straight part
We know that "one who foots the bill" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one who foots the bill = payer.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 PAYER mean?
PAYER noun- One who pays; specifically, the person by whom a bill or note has been, or should be, paid.
- A swaption which gives its holder the option to enter into a swap in which they pay the fixed leg and receive the floating leg.