Question about subclasses

I understand the concept of a gish subclass, a melee subclass that is for a spellcasting clas. is there a reversal of that? a spellcasting class for a melee/martial class?

how would one create something like that?

how many spell levels would they be granted?

I want to try and make one for a homebrew melee class, but I am a little lost on how this could be balanced