The best way to answer this question is in a way that turns your negatives into a positive. You never want to say anything that will emphasize weaknesses you may have. For example, to answer this question, you might say that you tend to be a perfectionist (which can be seen as a weakness sometimes), but in reality, it shows you pay attention to detail. If you don't have much law enforcement experience (if you just graduated from your law enforcement training, for example, and haven't worked in that field), you can simply say that despite not having a job in the field, you've completed your training and you are a fast learner and eager to learn, etc.
The weakness answer has three parts:
- Briefly qualify (weakness, not a flaw)
- State your weakness (the answer)
- Say what you're doing about it (improvement)
The best answer shows how you handle limitations and adversity (which everybody faces). Show how you maximize your effectiveness. In short, what is your improvement plan? Do you demonstrate ability to change and grow?
Do not respond with a smarty-pants non-answer like "I'm a perfectionist, so really my weakness is a strength" which doesn't answer the question and shows the interviewer the way you really handle adversity is to hide it, ignore it, or otherwise lie about it.
If you can't think of a "weakness" remember that the bottom half of all your listed "strengths" are your weaknesses (things that need improvement).