Grace under pressure (6)
(Other definitions for aplomb that I've seen before include "Self-confidence, assurance" , "Self confidence" , "Nonchalant confidence" , "Air of self-satisfied confidence" , "Cool self-assurance" .)
'grace under pressure' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for poise that I've seen before include "Balance; composure" , "Coolness and composure under strain" , "Aplomb" , "Sang-froid" , "Dignity; equilibrium" .)