Question

adverbial clause ' come back at ten o'clock'. ​

Answer 1

there is adverb phrase in this sentence, not adverb clause- at ten o clock (adverb phrase of time)

if you want to add adverb clause then :

come back when it is ten o clock

Explanation:

hope this is what you are looking for

Answer 2

Answer:

Come back when it is ten o' clock