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