English genetic material's in the box forever (9)
I believe the answer is:
eternally
'forever' is the definition.
(I know that forever can be written as eternally)
'english genetic material's in the box' is the wordplay.
'english' becomes 'e' (abbreviation).
'genetic material' becomes 'rna' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'the box' becomes 'telly' (both can mean a television set).
'rna' put into 'telly' is 'ternally'.
'e'+'ternally'='ETERNALLY'
(Other definitions for eternally that I've seen before include "Nearly let out forever" , "constantly" , "all the time" , "for good" , "Always and ever" .)