$filename is handled as a shell variable.

What about

ffmpeg -i clip.mp4 fr1/clip_%d.jpg -hide_banner 

or

$mp4filename=clip ffmpeg -i ${mp4filename}.mp4 fr1/${mp4filename}_%d.jpg -hide_banner 

?

Edit: Fixed variable reference as pointed out by @DonHolgo.

$filename is handled as a shell variable.

What about

ffmpeg -i clip.mp4 fr1/clip_%d.jpg -hide_banner 

or

$mp4filename=clip ffmpeg -i ${mp4filename}.mp4 fr1/${mp4filename}_%d.jpg -hide_banner 

?

Update: For use with gnu parallel, you can use parallel's -i option:

-i
Normally the command is passed the argument at the end of its command line. With this option, any instances of "{}" in the command are replaced with the argument.

The resulting command line could be as simple as

parallel -i ffmpeg -i {} fr1/{}_%d.jpg -hide_banner -- *.mp4 

if you can live with the extension in the output files.

Be aware that you may not actually want to run this in parallel on a traditional hard-disk as the concurrent i/o will slow it down.

Edit: Fixed variable reference as pointed out by @DonHolgo.

$filename is handled as a shell variable.

What about

ffmpeg -i clip.mp4 fr1/clip_%d.jpg -hide_banner 

or

$mp4filename=clip ffmpeg -i $mp4filename.mp4 fr1/$mp4filename_%d.jpg -hide_banner 

?

$filename is handled as a shell variable.

What about

ffmpeg -i clip.mp4 fr1/clip_%d.jpg -hide_banner 

or

$mp4filename=clip ffmpeg -i ${mp4filename}.mp4 fr1/${mp4filename}_%d.jpg -hide_banner 

?

Edit: Fixed variable reference as pointed out by @DonHolgo.