Pick source of radioactivity on particle (8)
I believe the answer is:
electron
'particle' is the definition.
(type of sub-atomic particle)
'pick source of radioactivity on' is the wordplay.
'pick' becomes 'elect' (I've seen this in another clue).
'source of' says to take the initial letters.
The initial letter of 'radioactivity' is 'r'.
'elect'+'r'+'on'='ELECTRON'
(Other definitions for electron that I've seen before include "One charged" , "Subatomic particle -- enrol etc (anag)" , "Part of atom" , "Particle found in all atoms" , "Charged subatomic particle" .)