Composure is seen in US writer (5)
I believe the answer is:
poise
'composure' is the definition.
(I know that poise is a type of composure)
'is seen in us writer' is the wordplay.
'seen in' indicates putting letters inside.
'us writer' becomes 'poe' (Edgar Allan Poe).
'is' put into 'poe' is 'POISE'.
(Other definitions for poise that I've seen before include "Grace; equilibrium" , "Graceful and elegant bearing in a person" , "cool" , "Dignified and self-assured manner" , "Dignity and assurance of manner" .)