The syntax: *mem_ptr_t<sys_page_attr_t> a* is preferred. So you could simply use:

a->attribute = 0;
a->page_size = 0;
a->access_right = 0;
a->pad = 0;

Look at other modules for more examples if you want:
(e.g. https://github.com/DHrpcs3/rpcs3/blob/master/rpcs3/Emu/SysCalls/Modules/cellFont.cpp#L375)