Such as Maggie Tulliver, oddly in love: a rarity! (3,2,1,7)
I believe the answer is:
one in a million
'a rarity' is the definition.
'such as maggie tulliver oddly in love' is the wordplay.
'such as maggie tulliver' becomes 'one in a mill' (character in The Mill on the Floss, George Eliot).
'oddly' indicates anagramming the letters.
'love' becomes 'o' (love means zero in tennis).
'in'+'o'='ino'
'ino' is an anagram of 'ion'.
'oneinamill'+'ion'='ONE IN A MILLION'
(Other definitions for one in a million that I've seen before include "someone very special? - with 24a" , "Wonderful person, almost unique?" , "Quite exceptional person" , "Gem; exceptional person" , "Rare or exceptional person or thing" .)