Knight stopping revision of a mutual description of the Fall? (8)
I believe the answer is:
autumnal
'description of the fall?' is the definition.
(I've seen this before)
'knight stopping revision of a mutual' is the wordplay.
'knight' becomes 'n' (chess abbreviation).
'stopping' indicates putting letters inside (stopping can mean blocking a gap).
'revision of' indicates anagramming the letters.
'a'+'mutual'='amutual'
'amutual' is an anagram of 'autumal'.
'n' put inside 'autumal' is 'AUTUMNAL'.
(Other definitions for autumnal that I've seen before include "in a particular part of the year" , "Seasonal" , "Of the season of mists and mellow fruitfulness" , "To do with the harvest season or fall" , "Characteristic of the fall" .)