Particle, negative, orbiting form of inert uranium (8)
I believe the answer is:
neutrino
'particle' is the definition.
(neutrino is a kind of particle)
'negative orbiting form of inert uranium' is the wordplay.
'negative' becomes 'no' (no is a kind of negative).
'orbiting' is an insertion indicator.
'form of' indicates anagramming the letters (another form of the same letters).
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'inert'+'u'='inertu'
'inertu' with letters rearranged gives 'eutrin'.
'no' going around 'eutrin' is 'NEUTRINO'.
(Other definitions for neutrino that I've seen before include "Small piece of matter" , "Minute item lacking mass" , "There's almost nothing to it!" , "Subatomic particle, now said to travel faster than light" , "This barely affects anything" .)