Depends on that one line. If the line is readable and concise by itself, the function may not be needed. Simplistic example:

<!-- language-all: java -->

 void printNewLine() {
 System.out.println();
 }

OTOH, if the function gives a good name to a line of code containing e.g. a complex, hard to read expression, it is perfectly justified (to me). Contrived example (broken into multiple lines for readability here):

 boolean isTaxPayerEligibleForTaxRefund() {
 return taxPayer.isFemale() 
 && (taxPayer.getNumberOfChildren() > 2 
 || (taxPayer.getAge() > 50 && taxPayer.getEmployer().isNonProfit()));
 }