Mother backed by her brothers in America (5,3)
I believe the answer is:
uncle sam
'brothers in america' is the definition.
'uncle sam' can be an answer for 'america' (I've seen this before). I'm not sure about the 'brothers in' bit.
'mother backed by her' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'backed' is a reversal indicator.
'by' means one lot of letters go next to another.
'her' becomes 'uncles' (I can't explain this - if you can you should believe this answer much more).
'ma' reversed gives 'am'.
'am' after 'uncles' is 'UNCLE SAM'.
(Other definitions for uncle sam that I've seen before include "The United States" , "Cotton on" , "Federal government" , "Personification of USA" , "US government personification" .)