Occurrence at Wimbledon featured in best-seller, unusually stylish writing (6,7)
I believe the answer is:
belles lettres
'stylish writing' is the definition.
(French term for fine or beautiful writing)
'occurrence at wimbledon featured in best-seller unusually' is the wordplay.
'occurrence at wimbledon' becomes 'let' (type of tennis serve).
'featured in' is an insertion indicator.
'unusually' indicates an anagram.
'best'+'seller'='bestseller'
'bestseller' anagrammed gives 'bellestres'.
'let' put within 'bellestres' is 'BELLES-LETTRES'.
(Other definitions for belles lettres that I've seen before include "literary studies" , "Literary works" , "writings" .)