Showing composure, one adopted an attitude outside (6)
I believe the answer is:
poised
'showing composure one adopted an attitude outside' is the definition.
I can't tell whether this defines the answer.
'one adopted an attitude outside' is the wordplay.
'one' becomes 'i' (Roman numeral).
'adopted' is an insertion indicator (inserted letters are welcomed or adopted).
'an attitude' becomes 'pose' (attitude can mean a pose or stance).
'outside' becomes 'd' (I am not sure about this - if you are sure you should believe this answer much more).
'i' going inside 'pose' is 'poise'.
'poise'+'d'='POISED'
Can you help me to learn more?
(Other definitions for poised that I've seen before include "Elegant, self-assured" , "Composed and self-assured" , "Elegant, self assured" , "Balanced or self-assured" , "Self-possessed or balanced" .)