Writer of a letter in sport, mischievously? (6)
I believe the answer is:
proust
'writer' is the definition.
(I know that Proust is a novelist)
'a letter in sport mischievously?' is the wordplay.
'a letter' becomes 'u' ().
'in' is an insertion indicator.
'mischievously?' indicates anagramming the letters (I've seen 'mischievously' mean this (I've seen this in other clues)).
'sport' with letters rearranged gives 'prost'.
'u' put into 'prost' is 'PROUST'.
'of' acts as a link.
(Other definitions for proust that I've seen before include "Famous French novelist in stupor" , "French novelist - stupor (anag)" , "French author" , "See 8" , "Marcel ---, Fr. author" .)