Answer:
You would really have to look into this yourself, especially considering the lack of specifics you provided, and research the copyright policy according to the song or whomever the song is owned by.
However, I can still give a bit of my knowledge for a general aspect, but there are a lot more factors you would need to look into since "famous song" is very vague and you didn't specify what you plan on using the song for.
So here's a general overview:
The usage of an intro or any element of a well-known song in your own song may generate copyright concerns. Original musical creations, including melodies, lyrics, and orchestration, are protected under copyright law. Using a large chunk of a copyrighted work without the owner's consent may be deemed an infringement.
To establish the legality of using a section of a popular song in your own song, examine the length of the sample, the relevance of the portion used, and if it qualifies as fair use. Fair use is a legal notion that permits the unrestricted use of copyrighted content for purposes such as criticism, commentary, parody, or education. However, deciding whether a certain use is fair use needs a case-by-case review, which might be subjective.
It is VERYY recommended to seek the assistance of a knowledgeable intellectual property attorney who can provide legal guidance based on your individual circumstances and applicable legislation in your region. They can assist you in understanding potential dangers and getting the essential rights or licenses.
Have fun w your song!