Form One initially chose a literary masterpiece (7)
I believe the answer is:
classic
'a literary masterpiece' is the definition.
(I've seen this before)
'form one initially chose' is the wordplay.
'form' becomes 'class' ('class' can be a synonym of 'form'**).
'one' becomes 'i' (Roman numeral).
'initially' indicates taking the first letters.
The first letter of 'chose' is 'c'.
'class'+'i'+'c'='CLASSIC'
(Other definitions for classic that I've seen before include "of enduring significance" , "Model" , "The best of its kind" , "Artistic work of the highest excellence" , "Serving as a standard - of the highest quality" .)