How would you write a riddle in which the answer is either a candle or a clock, containing alliteration,a caesura, and a kenning in it and has to be 4 to 8 lines long?
I am assuming you're not looking for someone to actually write the riddle for you. Instead, I'll give you some information regarding each of the literary devices, along with some thematically appropriate examples.
Alliteration involves repetition of a particular letter or sound at the beginning of a string of words. "Tick tocks tell time." is an example of alliteration. One does not need the entire sentence to follow the pattern; even two words can form alliteration.
A caesura is a natural pause within a line or phrase which is usually denoted with some form of punctuation. "Breaking News: Bernie's Bunsen burns bright tonight," contains a caesura right after the word "news". It is indicated by the colon. (It also contains alliteration.)
A kenning is a compound expression meant to represent a noun (or occasionally a verb.) Scholars disagree as to whether the kenning should be strictly metaphorical or whether it can be fairly literal. "Wax melter" would be a literal kenning describing a candle. Importantly, the noun being described should not be part of the kenning.
A great riddle is as fun to read as it is to solve. Good luck with the poem.