Company about to declare capital (5)
I believe the answer is:
cairo
'capital' is the definition.
(capital of Egypt)
'company about to declare' is the wordplay.
'company' becomes 'co' (abbreviation).
'about' means one lot of letters goes inside another.
'to declare' becomes 'air' (I can't justify this - if you can you should believe this answer much more).
'co' placed around 'air' is 'CAIRO'.
Can you help me to learn more?
(Other definitions for cairo that I've seen before include "City near the Nile delta" , "North African capital" , "Capital of Egypt" , "Al Qahira" , "Egypt's capital" .)