Giant company supported by America admits defeat (8)
I believe the answer is:
colossus
'giant' is the definition.
('colossus' can be a synonym of 'giant')
'company supported by america admits defeat' is the wordplay.
'company' becomes 'co' (abbreviation).
'supported by' means one lot of letters go next to another (letters go below - in down clue).
'america' becomes 'us' (abbreviation for the United States).
'admits defeat' becomes 'loss' (I can't justify this - if you can you should believe this answer much more).
'us' put after 'loss' is 'lossus'.
'co'+'lossus'='COLOSSUS'
Can you help me to learn more?
(Other definitions for colossus that I've seen before include "An immense statue, once a wonder in Rhodes" , "An immense statue - there was one at Rhodes" , "Abnormally powerful person" , "An immense state as once at Rhodes (8)" , "Something larger than life" .)