Urine not analysed for tiny particle (8)
I believe the answer is:
neutrino
'tiny particle' is the definition.
(type of subatomic particle)
'urine not analysed' is the wordplay.
'analysed' indicates anagramming the letters.
'urine'+'not'='urinenot'
'urinenot' with letters rearranged gives 'NEUTRINO'.
'for' is the link.
(Other definitions for neutrino that I've seen before include "Subatomic particle, now said to travel faster than light" , "Minute item lacking mass" , "Tiny thing not charged" , "Spinner" , "Small piece of matter" .)