Careless mistake (7)
I believe the answer is:
blunder
'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" .)