Is it allowed to ask for the meaning (author's intent) of a whole song? Or should I specify a time-frame/some lines?
This is acceptable, in my opinion. Some songs have meaning which are canon (they have been answered specifically by the artist as to what they're about). However, I would word it something like, "Is there a canonical meaning to [my song name] by [my artist name], and, if so, what is it?" This way, you're not asking for opinions (which is a gray area, but highly discouraged), but rather an interview or other written material where the song content is discussed by the author.