Single stage urethane can most certainly be cleared over.
It's done all the time and my preferred way.
Afterall, what is clear? it's urethane, the same type of paint,
just without the pigment (color).
It's a very strong durable way to paint at a lot less costs of most basecoats.
And most will agree that SS urethane will actually give you deeper
more intense color.
And if you do get your clear to thin after buffing,
(loosing the UV protection)
or chip the clear off in the future,
the urethane will hold up just fine.