Answer:
Prepare to go on stage once I tell you when the best time is.
Explanation:
An adverb clause is a group of related words that have at least a subject and a verb, and whose function is to act as an adverb, that is to say, to modify or add detail to adjectives, verbs or other adverbs. This type of clauses often answers questions such as "Why?", "How?", "When?", "Where?" and "In what circumstances?", and it often begins with subordinating conjunctions (once, as soon as, after, before, since) that link the adverb clause to another clause.
In the sentence provided, the adverb clause is "once I tell you when the best time is" as it modifies the main action "to prepare to go on stage", and answers the question "When should you prepare to go on stage?".
