(defun c:D10 () (command "_circle" pause "_Diameter" 10) (princ))
If you want to use some specific coordinates, use
(defun c:D10 () (command "_circle" "_non" '(10 20 0) "_Diameter" 10) (princ))
Another option
(defun c:somefunc ( / ) (entmake (list '(0 . "circle") (cons 10 '(1 1 0)) '(40 . 5.0))))
will draw circle with radius of 5 units at coordinate 1,1,0