Benefactor promoting new American centre (7)
I believe the answer is:
nucleus
'centre' is the definition.
(nucleus is a kind of centre)
'benefactor promoting new american' is the wordplay.
I cannot quite see how this works, but
'new' could be 'n' and 'n' is present in the answer.
'american' could be 'us' and 'us' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nucleus that I've seen before include "Core, centre" , "Centre of an atom" , "Tiny factor" , "Centre of growth" , "Central mass - kernel" .)