Eccentric not even going to dance (7)
I believe the answer is:
oddball
'eccentric' is the definition.
(an oddball is an eccentric person)
'not even going to dance' is the wordplay.
'not even' becomes 'odd' ('odd' is the opposite to 'even').
'going to' says to put letters next to each other.
'dance' becomes 'ball' (ball is a kind of dance).
'odd'+'ball'='ODDBALL'
(Other definitions for oddball that I've seen before include "Bold lad may be eccentric" , "Strange person (colloq.)" , "Weirdo" , "Bold lad (anag)" , "case" .)