Sit keeping one's composure (5)
I believe the answer is:
poise
'composure' is the definition.
(poise is a kind of composure)
'sit keeping one's' is the wordplay.
'sit' becomes 'pose' ('pose' can be a synonym of 'sit'**).
'keeping' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'pose' going around 'i' is 'POISE'.
(Other definitions for poise that I've seen before include "Aplomb" , "Balance; composure" , "Elegance" , "Grace; equilibrium" , "Dignity; equilibrium" .)