Extraordinary dormice: tail on one is ordinary (8)
I believe the answer is:
mediocre
'is ordinary' is the definition.
(similar in meaning)
'extraordinary dormice tail on one' is the wordplay.
'extraordinary' indicates an anagram.
'tail on' indicates one should take the final letters (the tail of the word).
The last letter of 'one' is 'e'.
'dormice' with letters rearranged gives 'mediocr'.
'mediocr'+'e'='MEDIOCRE'
(Other definitions for mediocre that I've seen before include "Rather poor" , "Run-of-the-mill" , "Fairly bad" , "Commonplace, not distinguished" , "Ordinary" .)