A sound and a flash (4)
I believe the answer is:
tick
I believe this clue is a double definition.
'a sound' is the first definition.
(tick is a kind of sound)
'a flash' is the second definition.
(thesaurus)
'and' is the link.
(Other definitions for tick that I've seen before include "Small parasite" , "Make noise like old clock" , "Arrangement for deferred payment" , "Parasitic insect or clicking sound" , "Instant - credit" .)