Don't vote and get a black mark (7)
I believe the answer is:
abstain
'don't vote' is the definition.
(I've seen this before)
'get a black mark' is the wordplay.
'get' means one lot of letters go next to another.
'black' becomes 'b' (as in pencil classifications).
'mark' becomes 'stain' ('stain' can be a synonym of 'mark'**).
'a' next to 'b' is 'ab'.
'ab'+'stain'='ABSTAIN'
'and' acts as a link.
(Other definitions for abstain that I've seen before include "Take no part" , "Choose not to consume" , "Don't drink" , "Spare" , "Cut it out!" .)