It's certainly doable but I wouldn't bother with a tour guide and an in-depth visit.
There is a train every half an hour from MXP to the city centre, and it takes around 45 mins. Take that, go to Duomo, have some lunch, stroll around the city and do a bit of shopping (if you can live with the prices!) before returning to MXP.
From your post it doesn't seem likely, but, if you fancy an even more leisurely day, you can always visit Busto Arsizio, which is the first stop on the train from MXP to Milan. It's a very pleasant town (pop: 80000), and only 7 minutes away from Malpensa! ^