The most common and most likely answer to this crossword clue is the 18 letter word SONG-AND-DANCE-MAN
I've seen this crossword clue in Financial Times
S
O
N
G
-
A
N
D
-
D
A
N
C
E
-
M
A
N
Buddy explains!
The straight part
We know that "entertainer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case entertainer = song-and-dance-man.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.