… and reviews vital instructions (3)
I believe the answer is:
dna
'vital instructions' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'and reviews' is the wordplay.
'reviews' says the letters should be written backwards (I've seen 'review' mean this).
'and' written backwards gives 'DNA'.
Can you help me to learn more?
(Other definitions for dna that I've seen before include "Genetic carrier" , "Substance in chromosomes that stores genetic information, used by police" , "Substance that stores genetic information - often used by police" , "Substance in chromosomes that stores genetic data" , "It carries genetic information" .)