It's considered precious by most involved in DNA unit (8)
I believe the answer is:
gemstone
'it's considered precious' is the definition.
(I've seen this before)
'most involved in dna unit' is the wordplay.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'in' indicates putting letters inside.
'dna unit' becomes 'gene' (I've seen this before).
'most' is an anagram of 'msto'.
'msto' going inside 'gene' is 'GEMSTONE'.
'by' is the link.
(Other definitions for gemstone that I've seen before include "Piece of crystalline rock as jewellery" , "Emerald or ruby" , "Sparkler" , "Eg, diamond or ruby" , "Eg, sapphire" .)