Old writer featuring in upcoming total collected works (7)
I believe the answer is:
omnibus
'collected works' is the definition.
(I've seen this in another clue)
'old writer featuring in upcoming total' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'writer' becomes 'nib' (something which writes).
'featuring in' indicates putting letters inside.
'upcoming' says the letters should be written in reverse (in a down clue coming up is backwards).
'total' becomes 'sum' (synonyms).
'sum' reversed gives 'mus'.
'nib' going within 'mus' is 'mnibus'.
'o'+'mnibus'='OMNIBUS'
(Other definitions for omnibus that I've seen before include "Collection of books reprinted in one volume" , "reading this" , "Compendium volume" , "Book - public transport" , "show?" .)