The first Spanish monolingual dictionary we would like to recommend is the one offered by the Real Academia de la Lengua Española (Royal Spanish Academy), an official organisation that regulates, changes and adds new terms to the Spanish language.
Following this link, you will find the Spanish Language Dictionary, the Pan-Hispanic Dictionary of Doubts, the Spanish Legal Dictionary, the Historic Spanish Dictionary and the Dictionary of Americanisms.
The Pan-Hispanic Dictionary of Doubts is a resource with over 7000 entries giving answers to clarify and argue the most common doubts surrounding the use of Spanish whether orthographic, morphological, lexical-semantic or phonological. However, we find two types of articles in this dictionary: thematic, covering general topics such as accents or the use of punctuation, and non-thematic, covering specific words or linguistic doubts.
You can find it at the link below:
For monolingual queries, we also have the Fundación del Español Urgente or Fundéu. This foundation publishes daily recommendations on the proper use of Spanish, based on doubts detected in mainstream media, requests from users through social media or the standard use of the language.
Link to Fundéu:
This Foundation is easily accessible to anyone and, if you don’t find an article in the database that answers your query, you can always ask them on Twitter, Facebook or Instagram and your question will be answered promptly.
For more information, you can visit the website of the National Library of Spain that offers an exhaustive article with several resources that you can use as reference: