The most common and most likely answer to this crossword clue is the 12 letter word CLAPPERBOARD
I've seen this crossword clue in The Daily Telegraph
C
L
A
P
P
E
R
B
O
A
R
D
Buddy explains!
The straight part
We know that "signal for action" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case signal for action = clapperboard.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 CLAPPERBOARD mean?
CLAPPERBOARD noun- A device used in film production, having hinged boards that are brought together with a clap, used to synchronize picture and sound at the start of each take of a motion picture or other video production.