Stupid mistake about routine (7)
I believe the answer is:
brutish
'stupid' is the definition.
(thesaurus)
'mistake about routine' is the wordplay.
'mistake' becomes 'bish' (I've seen this in other clues).
'about' is an insertion indicator.
'routine' becomes 'rut' (rut is a kind of routine).
'bish' going around 'rut' is 'BRUTISH'.
(Other definitions for brutish that I've seen before include "Rough and uncivilised" , "Crude" , "Senselessly cruel" , "Bestial" , "Animal" .)