It is always best to be open and honest, especially when potential references are brought up, and the interviewer could actually call them and ask this very question. Consider what feedback or area of improvement was recommended to you by your former boss. Of course, do not share something that would be a deal-breaker for this new job, but something that can be improved upon with a solid plan in time. Instead of defending your ability to do whatever was mentioned, share how you are working on this weakness. This will communicate to the interviewer that you are teachable and humble enough to accept feedback from a superior. Be sure to mention what actionable steps you are taking to thoughtfully and continually improve your abilities on the job.