Keep us up to the mark (7)
I believe the answer is:
sustain
'keep' is the definition.
(synonyms)
'us up to the mark' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'to the mark' becomes 'stain' ('stain' can be a synonym of 'mark').
'us' reversed gives 'su'.
'su'+'stain'='SUSTAIN'
(Other definitions for sustain that I've seen before include "Uphold, strengthen" , "Nurish" , "Undergo" , "Continue, prolong" , "Strengthen, support" .)