Cakewalk doesn't provide reasonable content manipulation functions in Surface API. But you can send keyboard shortcuts ('Keyboard' action). So after you focus (function) track, you can send ',' key to select the track. From my observations, that also selects all clips. And so you can send "Del" to delete them.
Note that the result is as "stable" as keyboard shortcuts. F.e. you may want to append send ',' to deselect the track. Also all clips are selected and deleted only in case there is no time selection.