Keep you and me up above the mark (7)
I believe the answer is:
sustain
'keep' is the definition.
('sustain' can be a synonym of 'keep')
'you and me up above the mark' is the wordplay.
'you and me' becomes 'us'.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'above' is a charade indicator (letters next to each other).
'the mark' becomes 'stain' ('stain' can be a synonym of 'mark'**).
'us' backwards is 'su'.
'su'+'stain'='SUSTAIN'
(Other definitions for sustain that I've seen before include "Support, keep going" , "Strengthen physically" , "Continue, prolong" , "Strengthen; suffer" , "Give food to" .)