Eccentric uncle working to find subatomic particle (7)
I believe the answer is:
nucleon
'subatomic particle' is the definition.
(nucleon is a kind of subatomic particle)
'eccentric uncle working' is the wordplay.
'eccentric' indicates an anagram.
'working' becomes 'on' (e.g. a machine is working or on).
'uncle' anagrammed gives 'nucle'.
'nucle'+'on'='NUCLEON'
'to find' is the link.
(Other definitions for nucleon that I've seen before include "Particle" , "fundamental constituent" .)