The most common and most likely answer to this crossword clue is the 5 plus 4 letter word GUEST BEER
I've seen this crossword clue in The Times
G
U
E
S
T
B
E
E
R
More clues leading to the result GUEST BEER
Buddy explains!
The straight part
We know that "not regularly seen at the bar" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not regularly seen at the bar = guest beer.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.