built.sbt

TaskKey[Unit]("cleanCompile") <<= {
  import Step._
  val go = 
    for {
      _ <- doClean
      _ <- doCompile
    } yield ()
  go.run
}

Step.scala

import sbt._
import Keys._

case class Step[A](run: Def.Initialize[Task[A]]) {
  def map[B](f: A => B): Step[B] =
    Step[B](Def.taskDyn {
      Def.task { f(run.value) }
    })
  def flatMap[B](f: A => Step[B]): Step[B] =
    Step[B](Def.taskDyn {
      f(run.value).run
    })
}

object Step {
  val doClean = Step(Def.task(clean.value))
  val doCompile = Step(Def.task(compile.in(Compile).value))
}