It's capital using mail order (4)
I believe the answer is:
lima
'it's capital' is the definition.
(Lima is an example)
'mail order' is the wordplay.
'order' indicates anagramming the letters.
'mail' anagrammed gives 'LIMA'.
'using' is the link.
(Other definitions for lima that I've seen before include "Send mail out to S America" , "city (like 22)" , "Mail to Peru's capital" , "Edible bean; capital city" , "Mail to Peru'd capital" .)