Expanding a macro within a string?

Sorry, this seems like it must be covered in the
docs, but I spent a few hours off & on searching, without luck.

I want to do something like:

define(XXX,ZZZ)

then expand XXX within a string:

someXXXstring => someZZZstring

Obviously, XXX would require some kind of delimiter:

some[XXX]string => someZZZstring.

I managed to do half of what I want;

defn(`XXX')string => ZZZstring

Is there any way to get the full monty?

Thanks,

Paul