I use the following which gives me words and I then pick out what I think is appropriate so sorry can't answer your question which is why i put the " ? "
http://www.onelook.com/
Just an afterthought, when an answer is hyphenated does Mephisto indicate hyphens in the number of letters (I can't remember)? If not then heart-to-heart would be 12.
Hyphenated words and phrases in Mephisto are treated as one word hence the solution here is clued as 12 letters. 5-2-5 would make it too easy, I suppose.