So you want to start a new project with an audio file which you dont know it´s tempo though it has a fixed tempo.
Tempo detection shows something like this:
but you want to work with a fixed tempo not with this tempo track.
I know there are other ways to do it but I share my way:
1- Insert the file in the project and run Tempo detection - analize
2- Open the sample editor, zoom and move the “S” to the beggining
3- Deactivate tempo track
4- Scroll tempo in transport bar so that the tempo line places in an average tempo
5- Move the audio event so that the start (“S”) matches a bar start, with Snap to grid on.
6- Now test with metronome activated, increase or decrease tempo from transport bar to fine tune and match the event start to a bar again every time.
7- When done delete tempo and signature tracks.
It seems complicated but it takes less than a minute in real time.
EDIT: Another way that I forgot and is simpler is:
1- Insert the file in the project and run Tempo detection - analize
2- Open the sample editor, zoom and move the “S” to the beggining
3- Select the audio event: Audio-Advanced-Set definition from tempo.
4- Delete tempo and signature tracks. Adjust tempo from transport bar.