I would love to plant a nice heavy nitrogen fixing cover crop in the citrus grove. But a thick mat of cover crop makes for a colder environment in the orchard. Which means that on a very frosty night it can make the difference between the fruit and even trees being damaged or not. A bare orchard floor gives up heat to the trees more and provides a warmer environment.
What I have done is to cultivate the floor between the citrus trees late (last week) and then sown a thin crop of Rapini. We make get a cut for our Eatwell Boxes from this. Even if not it will provide flowers late in the winter to feed the bees.