The output is attached. Its last line is
Let's reference it: theorem 1.2.2.
But it better be
Let's reference it: Def. und Thm. 1.2.2.
How to get it with `\cref` from `cleveref` automatically?
Loading `ntheorem` is unfortunately a no-go (way too antique, way too many bugs). Loading `amsthm` ist also opposed to, especially if you wish to explicitly typeset your own QED symbols that depend on the enclosing environments. In any case, my attempts to solve the problem with any of the two packages failed.
The same question has been opened at http://golatex.de/viewtopic,p,106043.html and http://texwelt.de/wissen/fragen/24620/s ... t-cleveref