Nation or third sector group supporting firm (5)
I believe the answer is:
congo
'nation' is the definition.
(Congo is an example)
'third sector group supporting firm' is the wordplay.
'third sector group' becomes 'ngo' (I am not sure about this - if you are sure you should believe this answer much more).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'firm' becomes 'co' (a firm is a company).
'ngo' put after 'co' is 'CONGO'.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for congo that I've seen before include "African republic" , "Wide river" , "Former name of Zaire" , "African runner" , "former African nation" .)