Eccentric learner one in Paris with a nervous twitch (7)
I believe the answer is:
lunatic
'eccentric' is the definition.
'learner one in paris with a nervous twitch' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'one in paris' becomes 'un' ('one' in French).
'with' says to put letters next to each other.
'nervous twitch' becomes 'tic' (involuntary twitch in the body).
'l'+'un'+'a'+'tic'='LUNATIC'
(Other definitions for lunatic that I've seen before include "crazed?" , "Crazy" , "Crackers or nuts" , "Foolish person" , "Nutcase" .)