Black line below boob (7)
I believe the answer is:
blunder
'boob' is the definition.
('blunder' can be a synonym of 'boob')
'black line below' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'line' becomes 'l'.
'below' becomes 'under' ('under' can be a synonym of 'below').
'b'+'l'+'under'='BLUNDER'
(Other definitions for blunder that I've seen before include "A clumsy mistake or movement" , "Clumsy error" , "Gross mistake" , "A stupid 1 down" , "Stupid mistake" .)