Composure one’s discovered in American writer (5)
I believe the answer is:
poise
'composure' is the definition.
(poise is a kind of composure)
'one's discovered in american writer' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'discovered in' means one lot of letters goes inside another.
'american writer' becomes 'poe' (Edgar Allan Poe).
'is' put inside 'poe' is 'POISE'.
(Other definitions for poise that I've seen before include "Aplomb" , "cool" , "Coolness and composure under strain" , "Elegance" , "Graceful and elegant bearing in a person" .)