Bad mistake (6)
(Other definitions for howler that I've seen before include "Glaring error" , "Bad blunder - type of monkey" , "Serious avoidable mistake" , "Schoolboy error - wolf?" , "A foolish mistake, sounds noisy" .)
'bad mistake' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for clanger that I've seen before include "Big mistake (when dropped)" , "Drop this to make a blunder" , "One ringing" , "dropped by accident?" , "noise that rings in the ear" .)
(Other definitions for blunder that I've seen before include "Clumsy mistake, gaffe" , "Embarrassing mistake" , "Stupid mistake" , "Clumsy error" , "Careless mistake" .)