Keats's 'season of mists and mellow fruitfulness' (6)
I believe the answer is:
autumn
(Other definitions for autumn that I've seen before include "addressed by Keats" , "Browning happened then" , "'O wild West Wind, thou breath of . . . . . . 's being' (Shelley)" , "Season of mists and mellow fruitfulness, according to Keats" , "Fall in North America" .)