Careless mistake (7)
'careless mistake' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for blunder that I've seen before include "Clumsy error" , "Clumsy mistake, gaffe" , "A stupid 1 down" , "Embarrassing mistake" , "Careless mistake" .)
(Other definitions for slip-up that I've seen before include "Minor inadvertent mistake" , "Minor lapse" , "Small error" , "Mistake, blunder" .)