this?

gistfile1.clj

user> (def a [[1 2] [3 4] [5 6]])
#'user/a

user> (def b [[1 7] [2 8] [5 6]])
#'user/b

user> (map (fn [[a b] [c d]]
             (if (= a c)
               [b d]))
           a
           b)
([2 7] nil [6 6])


user> (filter (comp not nil?) (map (fn [[a b] [c d]]
             (if (= a c)
               [b d]))
           a
           b))
([2 7] [6 6])




user> (for [[x y] a
            [u v] b
            :when (= x u)]
        [y v])


([2 7] [6 6])