Support American back on spot (7)
I believe the answer is:
sustain
'support' is the definition.
('sustain' can be a synonym of 'support')
'american back on spot' is the wordplay.
'american' becomes 'us'.
'back' is a reversal indicator.
'on' means one lot of letters go next to another.
'spot' becomes 'stain' ('stain' can be a synonym of 'spot'**).
'us' written backwards gives 'su'.
'su'+'stain'='SUSTAIN'
(Other definitions for sustain that I've seen before include "Uphold, strengthen" , "Endure" , "Continue, prolong" , "Nurture, uphold" , "Prolong, support" .)