The most common and most likely answer to this crossword clue is the 17 letter word COCK-A-DOODLE-DOO
I've seen this crossword clue in The Times
C
O
C
K
-
A
-
D
O
O
D
L
E
-
D
O
O
More clues leading to the result COCK-A-DOODLE-DOO
Buddy explains!

The straight part
We know that "wake-up call" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case wake-up call = cock-a-doodle-doo.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 COCK-A-DOODLE-DOO mean?
COCK-A-DOODLE-DOO noun- The cry of the rooster.
- Imitative of the cry of the rooster.