The most common and most likely answer to this crossword clue is the 11 letter word STROBOSCOPE
I've seen this crossword clue in The Times
S
T
R
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B
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Buddy explains!

The straight part
We know that "flasher" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flasher = stroboscope.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.
Crossword Answer definitions
STROBOSCOPE noun- Instrument for studying or observing periodic movement by rendering a moving body visible only at regular intervals.
- A lamp that produces short bursts of light that synchronizes with a camera shutter for photographing fast-moving objects.
- A photograph produced by such a machine.