A tiny speck (4)
I believe the answer is:
mote
'a tiny speck' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mote that I've seen before include "A particle of dust" , "Spot" , "Tiny piece, speck" , "Dust speck; sounds like watery defence" , "'Particle, speck (4)'" .)