The most common and most likely answer to this crossword clue is the 8 plus 7 plus 6 letter word WOLFGANG AMADEUS MOZART
I've seen this crossword clue in Independent
W
O
L
F
G
A
N
G
A
M
A
D
E
U
S
M
O
Z
A
R
T
Buddy explains!
The straight part
We know that "Barman" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case barman = wolfgang amadeus mozart.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.