gistfile1.txt

def lineToOffer(line: String, headers: List[String]) : Validation[NonEmptyList[String], Offer] = {
  ({commaSplit(_)} andThen {extractFields(_: List[String], headers)} apply line) :->
    {(v) => Offer(v._1, v._2, v._3, v._4)}
}

def commaSplit(l: String): String => List[String] = l.split(",").toList

def extractFields(x: List[String], headers: List[String]): Validation[NonEmptyList[String], (String, String, String, String)] = {
    notEmpty(x(headers.indexOf("offer_name")), "No offer name specified on line:" + x.mkString(","))) <|***|> (
      notEmpty(x(headers.indexOf("email")), "No email specified on line:" + x.mkString(",")),
      notEmpty(x(headers.indexOf("uuid")), "No uuid specified on line:" + x.mkString(",")),
      notEmpty(x(headers.indexOf("reward_name")), "No uuid specified on line:" + x.mkString(","))
  )
}

case class Offer(offerName: String, email: String, uuid: String, rewardName: String)

You can leave off some of that syntax - e.g. write (g _ andThen f)(x)
Maybe write extractFields in curried form, with args reversed, e.g. extractFields(headers:List)(x:List) = ..., and then you could write (g _ andThen extractFields(headers))
Also, if you declare functions with val f = (x:...) =>... instead of def f(x:...) = ..., you can leave off the trailing _ in those cases.