Jack and Mark do not vote (7)
I believe the answer is:
abstain
'do not vote' is the definition.
'jack and mark' is the wordplay.
'jack' becomes 'ab' (both can mean a sailor).
'and' says to put letters next to each other.
'mark' becomes 'stain' (synonyms).
'ab'+'stain'='ABSTAIN'
(Other definitions for abstain that I've seen before include "not come down on either side" , "Don't indulge" , "Stay neutral, in a way" , "Keep away from drink" , "Formally decline to vote" .)