www.ipso.co.uk/complain/complaints-form/
IPSO will publicise how many complaints they receive.
If everybody who thinks what Clarkson wrote was unacceptable, even if he dislikes MM (and he's under no obligation to like her) contacts IPSO, then it sends a message. And may dissuade editors from signing of on vileness like this in future.
Worth a try, surely.
If people remain quiet, it looks like it's ok.
Complaining to IPSO is better than tweeting your opinion, because it doesn't publicise the paper or Clarkson. Both of them relish the publicity.