Half a colon (3)
I believe the answer is:
dot
'half a colon' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dot that I've seen before include "Absolutely punctual, on it" , "Morse symbol; spot" , "'Spot, tiny mark (3)'" , "Spot, speck" , "scatter" .)