How do you approach persona definition when you are targeting freelancers or small business web development companies? The market is fragmented so it gets difficult to define a persona and its attributes.

I don’t have any direct experience in this sector, but I’d encourage you to start by better understanding the segments of your market.  Whether through interviews, surveys, or product usage analysis (ideally some combination), you’ll need to identify the common threads between segments of your seemingly-fragmented base, and then create personas based on those segments.  Some dimensions to consider that span across job titles/industries/buyer types: “jobs to be done” (see Clayton Christensen’s work), decision criteria, substitutes, level of pain, willingness to pay, etc.

Disclaimer - I've never marketed to this segment. 

That said, I find it very hard to believe that you can't pick our four or five comon archetypes among your buyers. You're solving a few common problems. What are those? How are you solving their problems? Go out and talk to 20, 30, 50 customers. If you can't pick out four or five common patterns among your buyers, you might want to consider whether this is a market you even want to be in.