Related Clues:

• Dissenter
• Oppositional prefix
• One kept in the cooler
• One side of a debate
• Negative vote
• Contrary
• Neophyte
• Prefix with lock or knock
• Con man?
• He's against it
• Prefix for disestablish-mentarianism
• Bad-mouths
• Disapproving
• Member of the other side
• Start to knock?