Glaring mistake (6)
I believe the answer is:
howler
'glaring mistake' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for howler that I've seen before include "Grievous error" , "Serious avoidable mistake" , "A foolish mistake, sounds noisy" , "Bad blunder - type of monkey" , "A glaring mistake, sounds vocally noisy" .)