@Stephen Schippers I am trying to create a symbol for a subcircuit(in .spice file) using the spice_sym_def attribute as shown in the picture...but it somehow doesn't seem to work(not appearing in the top level netlist). Any help would be appreciated

xschem has a test schematic examples/test_ne555.sch, the ne555 symbol has a spice_sym_def attribute containing the .include line. It shows up in the netlist. If it does not work for you try to update xschem.