Is it possible to do a watch on this and watch for just the value of the field a changing
Sure.
If you want to watch only for value a you can define ng-change for (suppose a is a input) input a.
HTML
<div ng-controller="Controller">
<div ng-repeat="val in abc">
<input type='text' ng-model='val.a' ng-change="onAChange(val.a)">
<input type='text' ng-model='val.b'>
</div>
</div>
JS
$scope.onAChange = function(mod){
console.log(mod);
};
Demo 1 Plunker
Other way, to $watch on all object abc and write "deep" watch, something like:
$scope.$watch(function () {
return $scope.abc[0].a;
},
function (newValue, oldValue) {
if(newValue == oldValue){return;}
console.log(newValue, oldValue);
}, true);
Demo 2 Plunker
As a side note:
You don't need var before $scope.abc.
