Use a variable to store your current layer first. Get it using getvar and setvar:
(defun c:rc ( / ly)
(setq ly (getvar "clayer"))
(command "-LAYER" "S" "rev 000 all" "tr" "50" "" "" "revcloud" "A" "700" "700" "o" pause "")
(setvar "clayer" ly)
(princ)
)