Ruin note about subatomic particle (8)
I believe the answer is:
neutrino
'subatomic particle' is the definition.
(neutrino is a kind of subatomic particle)
'ruin note about' is the wordplay.
'about' indicates anagramming the letters.
'ruin'+'note'='ruinnote'
'ruinnote' with letters rearranged gives 'NEUTRINO'.
(Other definitions for neutrino that I've seen before include "This barely affects anything" , "Small piece of matter" , "Subatomic particle, now said to travel faster than light" , "There's almost nothing to it!" , "Tiny thing not charged" .)