Old writer involved in problem over collected works (7)
I believe the answer is:
omnibus
'collected works' is the definition.
(thesaurus)
'old writer involved in problem over' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'writer' becomes 'nib' (something which writes).
'involved in' indicates putting letters inside.
'problem over' becomes 'mus' (I can't explain this - if you can you should believe this answer much more).
'nib' inserted inside 'mus' is 'mnibus'.
'o'+'mnibus'='OMNIBUS'
(Other definitions for omnibus that I've seen before include "Extended programme" , "Volume containing several books" , "Compound volume" , "Book containing several works by same author" , "Several books in one" .)