I think you have two options here.  

1 - Add an invocation of this fn as a top level call to each namespace that uses JSON
2 - Add an invocation of this fn at the top level of this ns

1 is more explicit, you require the fn and invoke it everywhere it's used.  2 is more subtle but probably more correct.  The function call happens once, but you would have to make sure to require the utils.clj everywhere (only the first one would matter).