Designate nucleus of wrong particle (8)
I believe the answer is:
electron
'particle' is the definition.
(type of sub-atomic particle)
'designate nucleus of wrong' is the wordplay.
'designate' becomes 'elect' (I've seen this in another clue).
'nucleus of' means to look at the middle letters (nucleus can mean the centre of something).
The central letters of 'wrong' are 'ron'.
'elect'+'ron'='ELECTRON'
(Other definitions for electron that I've seen before include "One's charged" , "Particle found in all atoms" , "Negatively charged particle" , "There's not much to it" , "Small matter" .)