Mark follows sailor in refrain (7)
I believe the answer is:
abstain
'refrain' is the definition.
(I know that refrain can be written as abstain)
'mark follows sailor' is the wordplay.
'mark' becomes 'stain' (synonyms).
'follows' means one lot of letters go next to another.
'sailor' becomes 'ab' (abbreviation for able seaman).
'stain' put after 'ab' is 'ABSTAIN'.
'in' acts as a link.
(Other definitions for abstain that I've seen before include "Go without" , "Refuse to vote" , "Avoid" , "not come down on either side" , "Refuse to imbibe" .)