Greeting welcoming a small number in Asian capital (5)
I believe the answer is:
hanoi
'asian capital' is the definition.
(capital of Vietnam)
'greeting welcoming a small number' is the wordplay.
'greeting' becomes 'hi' (hi is a kind of greeting).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'small number' becomes 'no' (abbreviation; 'small' asks for an abbreviation).
'a'+'no'='ano'
'hi' placed around 'ano' is 'HANOI'.
'in' is the link.
(Other definitions for hanoi that I've seen before include "Asian capital on the Red River" , "foreign city" , "Vietnamese city" , "Capital city of Vietnam" , "Vietnamese capital" .)