Stay away from a black spot (7)
I believe the answer is:
abstain
'stay away from' is the definition.
'a black spot' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'spot' becomes 'stain' (synonyms).
'a'+'b'+'stain'='ABSTAIN'
(Other definitions for abstain that I've seen before include "Refuse to vote" , "Choose not to consume" , "Forego,do without" , "go on wagon" , "Do without, as in lent" .)