Avoid a black spot (7)
I believe the answer is:
abstain
'avoid' is the definition.
(abstaining is a kind of avoiding)
'a black spot' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'spot' becomes 'stain' ('stain' can be a synonym of 'spot'**).
'a'+'b'+'stain'='ABSTAIN'
(Other definitions for abstain that I've seen before include "not come down on either side" , "Refrain (from drink)" , "Refuse to vote" , "Keep away from drink" , "Desist" .)