Ne plus ultra unusually dull (7)
I believe the answer is:
neutral
'dull' is the definition.
(thesaurus)
'ne plus ultra unusually' is the wordplay.
'plus' says to put letters next to each other.
'unusually' indicates an anagram.
'ultra' anagrammed gives 'utral'.
'ne'+'utral'='NEUTRAL'
(Other definitions for neutral that I've seen before include "On the fence" , "Indifferent; not in gear" , "Lacking a preference" , "Impartial; colour" , "Not siding with any parties to a dispute" .)