A first-class spy yarn; The Brotherhood of the Rose, by David Morrell. New York: A St. Martin's/Marek Book. 351 pp. $15.95.

; ''The Brotherhood of the Rose'' has everything, every ingredient necessary for a first-class thriller: high-tech weaponry, a conspiracy to worry defenders of democracy, a touch of sex, and an irrepressibly energetic plot.

At the center of David Morrell's sixth novel are Saul and Chris, two orphans raised by kindly old Eliot. Well, Eliot happens to be an intelligence type who has recruited more than one pair of orphans, trained them at an ''academy,'' and turned them loose to perform assassinations and bombings.

Saul and Chris catch on to Eliot's plan of world disruption, and so begins the Oedipal task of hunting Eliot down. Morrell is a fine craftsman, and it is obvious that he is stalking intrigue novelist Robert Ludlum. If he keeps writing books as good as this he'll soon catch up with the genre's foremost current practitioner.