The most common and most likely answer to this crossword clue is the 3 letter word TAB
I've seen this crossword clue in The Times
T
A
B
Buddy explains!
The straight part
We know that "Bill" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bill = tab.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 TAB mean?
TAB noun- A small flap or strip of material attached to something, for holding, manipulation, identification, opening etc.
- An ear.
- (by extension) A navigational widget, resembling a physical tab, for switching between documents or sets of controls.
- The page or form associated with such a navigational widget.
- (British army) A fast march or run with full kit.
- To affix with tabs; to label.
- A restaurant bill.
- Credit account, e.g., in a shop or bar; slate
- A space character that extends to the next aligned column, traditionally used for tabulation.
- To use the Tab key on a computer to advance the cursor or move the input focus, or on a typewriter to advance the carriage.
- A cigarette.
- A form of musical notation indicating fingering rather than the pitch of notes, commonly used for stringed instruments.
- (British slang) A student of Cambridge University.
- A tabloid newspaper.
- A tablet, especially one containing illicit drugs.
- A tableau curtain.