The most common and most likely answer to this crossword clue is the 4 plus 2 plus 3 plus 3 letter word PICK UP THE TAB
I've seen this crossword clue in The Daily Telegraph
P
I
C
K
U
P
T
H
E
T
A
B
More clues leading to the result PICK UP THE TAB
Buddy explains!
The straight part
We know that "Bill is welcome? Do this to show it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bill is welcome? do this to show it = pick up the 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.