I've just updated my Sony Xperia Z3 Compact to 5.0.2.
The clock on the lockscreen used to be like this:
but now it's stacked like this (two lines):
Is there a way to keep Lollipop but get the old one-line clock on the lockscreen back?
Yes, there is. You could use an alternate clock app, like Clock JB, that has the same widget's look and feel. I would recommend any of the Clock apps developed by Moblynx. They have the same look and feel, and can be customized to your liking.
But, if you can't add widgets to the lockscreen, you will have to resort to another lockscreen app. If you want to go for hardcore mods to make it look that way, I would suggest posting a question at the XDA forums. The mods over there might help you out.