Terrible mistake made by a monkey (6)
I believe the answer is:
howler
I believe this is a double definition.
'terrible mistake' is the first definition.
(a howler is a bad mistake)
'a monkey' is the second definition.
(I know that howler is a type of New World monkey)
'made by' acts as a link.
(Other definitions for howler that I've seen before include "Extremely funny joke or glaring blunder" , "Glaring mistake made by schoolboy" , "5 in 9 29 [REVEREND IN CHARLES DICKENS]" , "Bad blunder - type of monkey" , "Gaffe" .)