Don't vote using a black mark (7)
I believe the answer is:
abstain
'don't vote' is the definition.
(I've seen this before)
'a black mark' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'mark' becomes 'stain' ('stain' can be a synonym of 'mark'**).
'a'+'b'+'stain'='ABSTAIN'
'using' is the link.
(Other definitions for abstain that I've seen before include "Withhold one's vote" , "Formally decline to vote" , "Avoid" , "'Refrain from, voting or eating, say (7)'" , "Give backing to" .)