The most common and most likely answer to this crossword clue is the 18 letter word SPEAKÂ OFÂ THEÂ DEVIL
I've seen this crossword clue in The Guardian
S
P
E
A
K
Â
O
F
Â
T
H
E
Â
D
E
V
I
L
Buddy explains!
The straight part
We know that "an unexpected turn-up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an unexpected turn-up = speak of the devil.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.