### Clue: Notice director is overseeing nuclear researchers (7)
### Answer: DISCERN 
 ### Explanation: Definition: Notice
  D(abbrev. for director) + IS **placed above**(overseeing, in a down clue) CERN(originally $$C$$onseil $$E$$uropéen pour la $$R$$echerche $$N$$ucléaire  – French for European Council for Nuclear Research, but now Organisation Européenne pour la Recherche Nucléaire – European Organization for Nuclear Research, but retaining the old acronym.  Those scientists who accelerate atomic particles to smash into one another to discover even smaller particles, amongst other things.)