In this example the clusters are draw with a PictureMarkerSymbol and the features outside of the clusters are drawn with a SimpleMarkerSymbol:
<esri:Map>
<esri:FeatureLayer url="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/Earthquakes/EarthquakesFromLastSevenDays/FeatureServer/0">
<esri:clusterer>
<esri:WeightedClusterer>
<esri:symbol>
<esri:PictureMarkerSymbol/>
</esri:symbol>
</esri:WeightedClusterer>
</esri:clusterer>
<esri:symbol>
<esri:SimpleMarkerSymbol color="red"
size="15"
style="triangle"/>
</esri:symbol>
</esri:FeatureLayer>
</esri:Map>