Particle in a beam (4)
I believe the answer is:
mote
'particle in a beam' 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" , "Dust speck; sounds like watery defence" , "Tiny piece, speck" , "'Particle, speck (4)'" , "Small particle in a time" .)