There’s a hole in the bag now; an exceptional hole!
Literally: bag – inside – [possessive particle] – awl / auger / drill
Alternately: A person of exceptional ability will show through even if surrounded by large numbers of ordinary people. “The cream rises to the top.” An awl, or similar piercing tool, hidden inside a bag will poke out and show itself.
Notes: There are a number of four-character compounds that use 之 in its archaic function as the associative particle, and to this point I’ve avoided using them because it feels like cheating to me somehow. In this case I relented because of the remaining compounds that start with no, this was the only one referenced in all my usual sources, so I’m assuming that it’s the most commonly-used and accessible.
Modern Japanese orthography may see this written at times with の instead of 之, though. Other variants replace 錐 with 穎 (ei), “awn” or “glume” (both apparently spiky strictures on plants) or even with 類 (tagui), “type,” “kind [of thing],” although why I’m not sure. It doesn’t seem to have any associations with specifically sharp or poky things, and may simply be a common “misspelling” of the obscure 穎.
Compare and contrast 能ある鷹は爪を隠す.