You're right. A secondary risk is a risk that will happen as a result of responding to another risk. Although it is not that easy, you should account for secondary risks in your risk management plan.
Here's an example of a secondary risk:
Let's say you're managing a construction project, and one of the risks is that the supplier will not deliver the sand on time (it's a risk because in previous projects that same supplier had issues with on-time delivery of material). This is your primary risk.
In your risk management plan, you account for the above risk. Your chosen action, if the risk materializes, is to procure the sand of another supplier. Here's a potential risk that may ensue: What will happen if the quality of sand is different (e.g. of lower quality) than the one provided by the first supplier? This is a secondary risk and you should account for this risk as well (and include the appropriate response) in your RMP.