Possible Clues:

• Expert needs weight for elementary particle
• 19-Across component
• Something very small supporting a big mass?
• Three quarks
• Stable subatomic particle
• Particle, real or theoretical, of nucleus initially
• Nucleus occupant
• Nuclear particle with positive charge
• Back slogan opposing Queen's English? It's a little bit charged
• Sit up away from opportunist, as one is likely to be charged