The same logic used with an electric shaver would apply here. If it's not "razor-sharp", it's fine.

There are a few different definitions; Rabbi Heinemann of the Star-K, shlit'a, in the name of R' Moshe Feinstein [suggests][1] taking a whisker from someone's beard, and seeing whether the cutter can cut it the same way a razor would.

> Hagaon R' Moshe Feinstein זצ"ל , who permitted the use of electric
> shavers used a criteria similar to the shochet who would demonstrate
> how sharp his חלף (shechita knife) was.  To show the sharp edge of his
> knife, a shochet would take a hair from his beard, and holding the
> hair in one hand the shochet would see whether the חלף cut the
> dangling hair.  If the hair was severed, the shochet's steel passed
> the test.

My understanding is that a manual clipper works like a fancy set of scissors, is not "razor" sharp, and is therefore permissible.


  [1]: http://www.star-k.org/kashrus/kk-mitzvos-shavers.htm