The most common and most likely answer to this crossword clue is the 9 letter word AVAILABLE
I've seen this crossword clue in Independent
A
V
A
I
L
A
B
L
E
More clues leading to the result AVAILABLE
- Free article, almost useless, with large fit
- Woman apt to switch starters on menu
- A bishop put in a new villa close to shrine – that’s handy
- Free beer embodies very excellent party
- One science unit in a valley still to let?
- A viable alternative without the French is within reach
- First admission at Victoria and Albert liable to be made free
- I’ve a ball partying with drop of ale on tap
- Top grade face covering reportedly fitted for free
- Bachelor exiting villa with a babe, possibly unattached
Buddy explains!

The straight part
We know that "on tap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case on tap = available.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 AVAILABLE mean?
AVAILABLE adjective- Such as one may avail oneself of; capable of being used for the accomplishment of a purpose.
- Readily obtainable.
- Valid.
- Having sufficient power, force, or efficacy to achieve the purpose; availing, effective.
- Free to meet someone, speak on the telephone, enter a romantic relationship or the like.