Keep up! Mark supports American uprising (7)
I believe the answer is:
sustain
'keep up' is the definition.
('sustain' can be a synonym of 'keep up')
'mark supports american uprising' is the wordplay.
'mark' becomes 'stain' ('stain' can be a synonym of 'mark'**).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'american' becomes 'us'.
'uprising' shows that the letters should be reversed in order.
'us' back-to-front is 'su'.
'stain' put after 'su' is 'SUSTAIN'.
(Other definitions for sustain that I've seen before include "Strengthen, support" , "Give food to" , "Feed, nourish" , "Undergo" , "Support, keep going" .)