Silly mistake (4)
(Other definitions for boob that I've seen before include "Blunder (colloq.)" , "Mistake; breast (sl.)" , "Error" , "Slip" , "Embarrassing mistake" .)
'silly mistake' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for goof that I've seen before include "In slang, bungle or blunder" , "boob" , "Blunder, mistake (slang)" , "foolish error" , "(Make a) silly mistake" .)