Monkey's bad mistake (6)
I believe the answer is:
howler
This clue might be a double definition.
'monkey's' is the first definition.
(howler is a kind of monkey)
'bad mistake' is the second definition.
'howler' can be an answer for 'mistake' (I have seen 'Serious mistake' mean 'howler' so perhaps 'mistake' could also mean 'howler'). I'm not certain of the 'bad' bit.
Can you help me to learn more?
(Other definitions for howler that I've seen before include "A foolish mistake, sounds noisy" , "Glaring mistake made by schoolboy" , "Serious blunder" , "Serious avoidable mistake" , "A glaring mistake, sounds vocally noisy" .)