Chief of staff welcoming number one (4)
I believe the answer is:
main
'chief' is the definition.
('main' can be a synonym of 'chief')
'staff welcoming number one' is the wordplay.
'staff' becomes 'man' (manning is a kind of staffing**).
'welcoming' means one lot of letters goes inside another (in sense of 'inviting in').
'number one' becomes 'i' (Roman numeral).
'man' enclosing 'i' is 'MAIN'.
'of' acts as a link.
(Other definitions for main that I've seen before include "source of gas" , "Chief, primary" , "Ocean or principal pipe" , "Dominant" , "Chief - principal electricity cable" .)