You're confusing "clearing the fields" and "tending to the fields".
To clear the fields, you must make every chore in the evening ASAP (they begin at 17h40). Once it's over, Alex may start to clear the fields, unless they're doing someting else. You can join them to do it faster. There are 9 fields in total, so there are still 5 to clear once you've been offered partnership.
You can tend to the fields by yourself once enough are cleared and you've been offered partnership by Alex. If you can do that in a new game, either it's been changed or there's a bug.