On a tar mark, don't (7)
I believe the answer is:
abstain
'don't' is the definition.
Both the definition and answer are verbs in their base form.
Perhaps they are linked in a way I don't understand?
'on a tar mark' is the wordplay.
'on' means one lot of letters go next to another.
'a tar' becomes 'ab'.
'mark' becomes 'stain' (synonyms).
'ab' put next to 'stain' is 'ABSTAIN'.
Can you help me to learn more?
(Other definitions for abstain that I've seen before include "Refrain from doing something" , "Go without" , "Refrain (from drink)" , "Refrain, forbear" , "Keep away from drink" .)