There are several methods you can use to select clips in the Source Area.
The border of the clip proxy turns green indicating it is the Source Area focus clip and that it is selected.
The border of the last selected clip turns green; the borders of other selected clips turn yellow. The clip with the green border is both selected and the focus clip. Clips with the yellow border are only selected.
The gang to which the focus clip belongs is selected. For information on creating gangs, see Ganging Source Clips.