Communicate, communicate, communicate. The first thing you need to do is talk to your manager / boss / leader and explain why a spec is important for you and for him. Explain that if we have a spec then he wont have to deal with angry calls that the system doesn't do X,Y,Z. Explain that without a spec you'll be forced to assume.

If you're the manager, then it's your job to explain to the customer why a spec is important.

You've just gotta communicate my friend :) Managers, Team leaders and Clients (in my experience) are usually very good if you only communicate :)

I listened to a really good audio book recently and it talked about "The importance of communication, either the conversations you're not having, or not having well" ..

Chances are you as a developer are more than skilled enough to write the software, and your manager is more than capable of managing - but there's just a lack of (good) communication :) hope that helps