Company about to promote foreign capital (5)
I believe the answer is:
cairo
'capital' is the definition.
(capital of Egypt)
'company about to promote foreign' is the wordplay.
'company' becomes 'co' (abbreviation).
'about' is an insertion indicator.
'to promote' says the letters should be written backwards (I've seen 'promotes' mean this (in a down clue, letters go upwards)).
'foreign' becomes 'ria' (I can't explain this - if you can you should give a lot more credence to this answer).
'ria' back-to-front is 'air'.
'co' enclosing 'air' is 'CAIRO'.
Can you help me to learn more?
(Other definitions for cairo that I've seen before include "Port upstream from the Nile delta" , "Capital of Egypt" , "Egyptian capital" , "North African capital" , "Capital city on the Nile" .)