Thank you, iminoz. I finally worked that out for myself, and was just going in to delete the question when I saw you had already answered it. But for me, it was the hard slog of dumb elimination; for you, no doubt, it was plain sailing (in the pond with all those smews).
P.S. Why is the other 4-letter theme answer ZERO? I've scored that often enough in cricket, so know its thematic meaning, but I can't parse it. E or ER is presumably the "leading member of the establishment," but how is ZO or ZEO "cross"?