Small round mark (3)
I believe the answer is:
dot
'small round mark' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dot that I've seen before include "Long-ago year?" , "Spot, speck" , "Absolutely punctual, on it" , "Flyspeck: period in a web address" , "Internet punctuation mark" .)