Genuine one in screen adaptation (7)
I believe the answer is:
sincere
'genuine' is the definition.
(sincere can mean authentic or genuine)
'one in screen adaptation' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'adaptation' indicates an anagram.
'screen' with letters rearranged gives 'sncere'.
'i' going into 'sncere' is 'SINCERE'.
(Other definitions for sincere that I've seen before include "Open and genuine, not deceitful" , "Honest, frank" , "devout" , "Not hypocritical" , "Free from deception" .)