Not even I get terribly disheartened at this quirk (6)
I believe the answer is:
oddity
'this quirk' is the definition.
(I've seen this before)
'not even i get terribly disheartened' is the wordplay.
'not even' becomes 'odd' ('odd' is the opposite to 'even'**).
'get' says to put letters next to each other.
'disheartened' says to hollow out the word (remove centre letters).
'terribly' with its centre removed is 'ty'.
'odd'+'i'+'ty'='ODDITY'
'at' acts as a link.
(Other definitions for oddity that I've seen before include "Something strange or eccentric" , "Curio, rarity" , "Something strange or unusual" , "Twelve months" , "Peculiar thing or person" .)