<input type="text" ng-model="inputValue">
<button ng-click="clearInput()">Clear</button>

$scope.clearInput = function() {
  $scope.inputValue = '';
};

<form name="myForm">
  <input type="text" ng-model="inputValue" name="myInput">
  <button ng-click="clearInput()">Clear</button>
</form>

$scope.clearInput = function() {
  $scope.inputValue = '';
  $scope.myForm.$setPristine();
  $scope.myForm.$setUntouched();
};

<input type="text" ng-model="inputValue" id="myInput">
<button ng-click="clearInput()">Clear</button>

$scope.clearInput = function() {
  document.getElementById('myInput').value = '';
};