I don't think there is any easy tool for this. Very few fonts are designed to fit the metrics of another one. Arial was specially designed for that purpose, but I doubt any others were.

I doubt any others were

http://typophile.com/node/90341 and linked threads within.

It's technically possible to write a program to scan all of your own fonts and somehow determine a "best fit". There is no guarantee anyone ever copied Clarendon's metrics, so it's possible the best fit, when only regarding metrics, is (uh), let's say "Comic Sans". To name a perfectly random font of which I'm pretty sure you won't say "oh that would be close enough".

How would this best fit algorithm need to work? Mathematically closest for the average character width, or the entire font (so some chracters would be wide off and other would come close), or for a select set of characters?

In any case, since font metrics per character are not publicly available, such a piece of software would only work on the fonts that you already have.

If exact looks are more important to exact metrics, you could try Fivos Vilanakis' "Find my Font": Win / Mac: New tool "Find my Font" to identify fonts in images.

I think one could use ttx to extract the metrics for comparison, but the final cut is best done by eye.

what i'm trying to do is look for a font that could sit as "clarendon sans black"

http://www.myfonts.com/fonts/insigne/stratham/
http://typophile.com/node/32643
http://typophile.com/node/75510

Yanone has good tools for this sort of thing.

hhp