Using hash of plaintext as nonce in NaCl secretbox- bad idea?
In the traditional sense, a nonce is a number that is only used once (with the same key). Though occasionally there are other requirements. For example, we may require that the nonce be unpredictable. For more on this I recommend you read this answer and this answer.

Now, you don't go into enough detail on what you are doing to say whether or not you need unpredictability of the nonce. A hash of the plaintext (depending on how long the plaintext is) will not get you unpredictability. So, if you are using CBC mode, this is definitely risky.

More importantly, however, it is not clear whether you really get the "number used only once" property with your proposal. Will the same plaintext ever be sent encrypted with the same key? If the answer is that is it possible, then your idea is a bad thing to do. Exactly what the outcome would be depends on application level details (most importantly, what block cipher mode you are using) that you have not provided.