Wife in love mistreated one of five characters (5)
I believe the answer is:
vowel
'one of five characters' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'wife in love mistreated' is the wordplay.
'wife' becomes 'w' (abbreviation).
'in' is an insertion indicator.
'mistreated' indicates anagramming the letters.
'love' with letters rearranged gives 'voel'.
'w' put inside 'voel' is 'VOWEL'.
Can you help me to learn more?
(Other definitions for vowel that I've seen before include "A or e, say" , "Non-consonant" , "E.g. A, but not B" , "Letter representing sound" , "Not a consonant" .)