Wife captivated by playful love letter (5)
I believe the answer is:
vowel
'letter' is the definition.
(vowel is a kind of letter)
'wife captivated by playful love' is the wordplay.
'wife' becomes 'w' (abbreviation).
'captivated by' means one lot of letters goes inside another.
'playful' indicates anagramming the letters.
'love' with letters rearranged gives 'voel'.
'w' going inside 'voel' is 'VOWEL'.
(Other definitions for vowel that I've seen before include "A, E, I, O or U" , "O for one" , "Letter representing sound" , "E.g. A, but not B" , "Non-consonant" .)