On my machine, nothing I do will replace the clutter in default Square Objects. However, if I copy and build, say the small airfield, I can manipulate the clutter for the new object.
The Bug
All my efforts to change the clutter with textures in the tersrc folder alone, fail.
The clutter _E and _D will show in the TE for my custom object but
The clutter layout from the TE doesn't show in the game. It seems to pick up the underlying clutter with 'grass' growing up through everything, runways and buildings and the alpha.