Honest one in screen adaptation (7)
I believe the answer is:
sincere
'honest' is the definition.
(similar in meaning)
'one in screen adaptation' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'adaptation' indicates anagramming the letters.
'screen' is an anagram of 'sncere'.
'i' inserted inside 'sncere' is 'SINCERE'.
(Other definitions for sincere that I've seen before include "Without deceit, genuine" , "Genuine; deceit-free" , "Showing genuine feelings" , "Unfeigned, genuine" , "True" .)