Fat American backed 50% of diet (4)
I believe the answer is:
suet
'fat' is the definition.
(suet is a kind of fat)
'american backed 50 of diet' is the wordplay.
'american' becomes 'us'.
'backed' says the letters should be written backwards.
'50 of diet' becomes 'et' (I am not sure about this - if you are sure you should believe this answer much more).
'us' in reverse letter order is 'su'.
'su'+'et'='SUET'
Can you help me to learn more?
(Other definitions for suet that I've seen before include "Thin pancake" , "Edible fat used in cooking" , "Solid fat used in cooking" , "Edible animal fat" , "Fatty tissue" .)