أُوصِيكَ بِأَمْرٍ نَافِعٍ: احْفَظْ وَقْتَكَ.
I advise you about something beneficial: guard your time.
The shorter instruction “Guard your time” immediately states the directive. In the fuller wording, “something beneficial” first creates anticipation and attention, and the later expression specifies what was left general. The preliminary phrase can enhance the advice when that expectation serves the listener's situation.
Clarification after vagueness is not a label for any extended sentence. There should be a recognizable initial generality followed by the detail that resolves it; this semantic progression gives the addition its rhetorical function.