Descriptive prose not entirely an illegible mess? (4)
I believe the answer is:
blur
'an illegible mess?' is the definition.
(something blurry or indistinct)
'descriptive prose not entirely' is the wordplay.
'descriptive prose' becomes 'blurb' (piece of writing offering a preview of a story).
'not entirely' means to remove the last letter.
'blurb' with its final letter taken off is 'BLUR'.
(Other definitions for blur that I've seen before include "Make or become indistinct" , "Hazy patch" , "Smear, make indistinct" , "Make less distinct" , "Out-of-focus sight" .)