Flashing light damaged dictionary endlessly (9)
I believe the answer is:
indicator
'flashing light' is the definition.
(I've seen this before)
'damaged dictionary endlessly' is the wordplay.
'damaged' is an anagram indicator.
'endlessly' means to remove the last letter.
'dictionary' with its final letter taken away is 'dictionar'.
'dictionar' anagrammed gives 'INDICATOR'.
(Other definitions for indicator that I've seen before include "sign" , "Station information board" , "This shows" , "Signal" , "This will show" .)