Ruin note about elementary particle (8)
I believe the answer is:
neutrino
'elementary particle' is the definition.
(neutrino is a kind of elementary particle)
'ruin note about' is the wordplay.
'about' indicates anagramming the letters.
'ruin'+'note'='ruinnote'
'ruinnote' anagrammed gives 'NEUTRINO'.
(Other definitions for neutrino that I've seen before include "Minute item lacking mass" , "Tiny thing not charged" , "This barely affects anything" , "Subatomic particle, now said to travel faster than light" , "Uncharged elementary particle" .)