Related Clues:

• Missing a few marbles
• ___ to one's word
• Kind of duck
• Like all primes but one
• Legitimate
• Lacking a match
• Atypical
• Like some colors
• Quite unusual
• Unmatched, as socks
• Like Don Rickles' persona
• 'That's right'
• Out of the ordinary
• Lacking refinement
• Like Oscar and Felix's pairing