Well, the clerihew is AABB and the limerick is AABBA so you just have to leave off the last line. The rhythmic pattern of a clerihew is non-existent. In fact not sticking to any pattern is encouraged. This also means that you could just as well come up with a limerick without the last line:

An aspiring young poet named Zed
Took it into her pretty young head
To dash off some lines
And put them on signs.

It's the first line that kills most of them. Mine just snuck under the wire.