Also, after some further testing, I can confirm that reverting back to version 3.5.5 of the plugin fixes the issue.
Looking at the code changes, it looks as though square market place was introduced from 3.6.1 which seems to be causing the issue.
Also, as an update to this, we disabled all other payment methods to test the plugin in isolation. When only the clearpay method was selected, the error still persists:
Uncaught ReferenceError: Square is not defined
So it does not look as though this is a conflict with other plugins
