Ordinary dormice mistreated by English (8)
I believe the answer is:
mediocre
'ordinary' is the definition.
(I've seen this before)
'dormice mistreated by english' is the wordplay.
'mistreated' is an anagram indicator.
'by' means one lot of letters go next to another.
'english' becomes 'e' (abbreviation).
'dormice' is an anagram of 'mediocr'.
'mediocr'+'e'='MEDIOCRE'
(Other definitions for mediocre that I've seen before include "Nothing special" , "Of unexceptional quality" , "Ordinary" , "Pedestrian, second-rate" , "Average (but poor)" .)