var dimensionValue = 'SOME_DIMENSION_VALUE';
ga('set', 'dimension1', dimensionValue);

ga(function(tracker) {
  var clientId = tracker.get('clientId');
});

ga(function(tracker) {
  var clientId = tracker.get('clientId');
});
ga('set', 'dimension1', clientId);

var clientId = ga.getAll()[0].get('clientId');
ga('set', 'dimension1', clientId);

ga(function(tracker) {
  var clientId = tracker.get('clientId');
  ga('set', 'dimension1', clientId);
  ga('send', 'pageview');
});