(in a first-class game) A lower order batsman sent in when the light is dimming to play out the remaining overs of the day in order to protect more valuable batsmen for the next days play.

A nightwatchman is a lower-order batsman sent in when daylight is fading to protect specialist batsmen from having to bat in difficult conditions. Their job is to simply survive the remaining overs, not score quickly. This tactical move preserves better batsmen for the next day. The question describes this role accurately.