The dictionary definition is simply calling attention to a
certain grammatical construction in which you can say that Y is an X (
The husband is a fool.) as a subordinate thought within a bigger sentence by using the structure
X of a Y. (
that fool of a husband). X and Y are the terms in apposition.
a prince of a man, a fool of a husband, a devil of a problem
say the man is a prince (prince-like), the husband is a fool, the
problem is a devil (very difficult). And this kind of expression
in used within a larger sentence, thus:
Her grandfather is a prince of a man.
That fool of a husband has gambled away his paycheck again.
If we do that, we'll have a devil of a problem with the employees.
The
structure itself is not very productive in English. That is, they
are fairly fixed idiomatic expressions, and it's probably best just to
remember them when you encounter them. I wouldn't recommend
trying to make up new ones.
CJ