Does a field of study have to be defined in order for the concepts in it to be defined? I don’t see why.
Can’t you define concepts x,y,z, and then define the field of study as being “about x,y,z and the things related to them”?
I’m guessing you mean something different by “defining a field of study” (different from my default/initial interpretation of the phrase I mean), but I’m not sure what else you mean by it.