One signal in Morse code (3)
I believe the answer is:
dot
'one signal in morse code' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dot that I've seen before include "Point; speck" , "Small round spot" , "Internet punctuation mark" , "Flyspeck: period in a web address" , "One signal used in Morse code" .)