An Englishman collecting pounds, billion being collected (6)
I believe the answer is:
aplomb
'being collected' is the definition.
(aplomb is being collected or poised)
'an englishman collecting pounds billion' is the wordplay.
'an' becomes 'a'.
'englishman' becomes 'pom' (pom is an Australian term for an English person).
'collecting' means one lot of letters goes inside another.
'pounds' becomes 'L' (L can represent the monetary pound).
'billion' becomes 'b'.
'pom' enclosing 'l' is 'plom'.
'a'+'plom'+'b'='APLOMB'
(Other definitions for aplomb that I've seen before include "Cool self-confidence" , "Dignified self-confidence" , "Calm self-assurance and poise" , "Air of self-satisfied confidence" , "Self-possession - coolness" .)