The best thing might be to first comment on the answer and ask for clarifications in hope that the answer will be clarified by editing. Make sure to be specific in your comments, e.g.

Have you thought of writing the solution as a numbered list that covers all the steps? It might be easier to follow then. Also you forgot to mention step x which I had to figure out myself using your solution.

Also, remember the wiki aspect of the site. You can edit (or suggest an edit for) the answer to clarify it and make it more complete.

These ways of handling the issue are good in that they do not "steal" credit from the original answer and in that it does not flood a question with answers.

However, given that these approaches fail, I think there are situations where writing up your own clearer and more complete answer is justified.

The best thing might be to first comment on the answer and ask for what you feel is missing in hope that the answer will be changed by editing. Make sure to be specific in your comments, e.g.

Have you thought of writing the solution as a numbered list that covers all the steps? It might be easier to follow then. Also you forgot to mention step x which I had to figure out myself using your solution.

Also, remember the wiki aspect of the site. You can edit (or suggest an edit for) the answer to clarify it and make it more complete.

These ways of handling the issue are good in that they do not "steal" credit from the original answer and in that it does not flood a question with answers.

However, given that these approaches fail, I think there are situations where writing up your own clearer and more complete answer is justified.