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