Sits on the fence - tar leaves a mark (8)
I believe the answer is:
abstains
'sits on the fence' is the definition.
I can't tell whether this defines the answer.
'tar leaves a mark' is the wordplay.
'tar' becomes 'ab'.
'leaves a mark' becomes 'stains' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ab'+'stains'='ABSTAINS'
Can you help me to learn more?
(Other definitions for abstains that I've seen before include "'doesn't drink'" , "Chooses not to do" , "Avoids" , "Refrains" , "Declines to vote" .)