AVI is like MOV or MXF - basically it's a container that holds a CODEC for Audio and Video - A CODEC like ProRes or AAC, etc.
What you need to know is what CODECs are inside that AVI.
Have you tried downloading VLC and checking it?
Some AVIs can be converted by Quicktime 7 Pro if you can get the CODECs for it. [
support.apple.com]
Otherwise Perian might work [
www.perian.org] its no longer updated but can still be used. I do occasionally even on Mavericks.
Handbrake also may be able to convert it to a format you can use [
handbrake.fr]
For instant answers to more than one hundred common FCP questions, check out the LAFCPUG FAQ Wiki here : [www.lafcpug.org]