Is it possible to rewrite the output of this CSV item with Feeds Tamper?
I agree with MPD. Plugins are easy to write. That looks like a serialised Python object. Is that right?

If you want to avoid that then I think your only option is the regular expression tamper. Use a site that allows you too test your expression on your sample string to help get it right while being fuzzy enough to handle edge cases. Here's one.

https://regex101.com

This seems to work though I'm not 100% on why it only returns the last match so use with caution. Match is in $2.

\[{2}.+?\],\s*\[(u'(.+?)'\,*\s*)+\]{2}

Actually this might be more robust. Match is in $1.

\[{2}.+?\],\s*\[.+\,\s{0,1}u'(.+?)'\]{2}$

https://regex101.com/r/qQ0cC0/3