a lot depends on how detailed / accurate you want the separation between fg/bg to be.
if you don’t want too much detail / accuracy, then FCP7 can do the job.
the basics are pretty obvious
duplicate your clip/s into two layers, say V1, V2, (option short drag a clip upwards to dupe it)
apply a blur or defaces to V1, apply a garbage matte to V2.
it could be as simple as a diopter effect, where there’s a simple soft line delineating the bg,
or a more complex shape around your fg subject.
keep the shape loose so you don’t have to do a zillion key-frames.
see if you can still find the Paul Crisp garbage metres on line somewhere.
with those you can keyframe the position of your shape as a whole.
makes life a lot easier.
for anything more complex however, you’d need to go outside of FCP7.
Resolve is free, and has these sorts of tools.
cheers,
nick