Possible Clues:

• Dog's short tail docked
• Nasty mongrel
• Eye opener gives despicable person antidote
• It may keep its tail, even when one shortened it
• If what's in 23 across has a tail, cut it short
• Contemptible canine
• A low dog
• Aggressive dog needing a lot of restraint
• Old police force rejected disreputable type
• To have has its tail cut off would have made the brute shorter