Nourished American coming back dirty (9)
I believe the answer is:
sustained
'nourished' is the definition.
('sustain' can be a synonym of 'nourish')
'american coming back dirty' is the wordplay.
'american' becomes 'US' (United States).
'coming back' is a reversal indicator.
'dirty' becomes 'stained' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'us' reversed gives 'su'.
'su'+'stained'='SUSTAINED'
Can you help me to learn more?
(Other definitions for sustained that I've seen before include "held up" , "bore" , "Kept (something) going" , "Kept going, supported" , "prolonged" .)