Mentions further that many are in sad disarray (4)
I believe the answer is:
adds
'mentions further' is the definition.
(I've seen this in another clue)
'many are in sad disarray' is the wordplay.
'many' becomes 'D' (Roman numeral for 500).
'are in' means one lot of letters goes inside another.
'disarray' is an anagram indicator.
'sad' anagrammed gives 'ads'.
'd' inserted inside 'ads' is 'ADDS'.
'that' is the link.
(Other definitions for adds that I've seen before include "Puts (numbers) together" , "Finds the sum (4)," , "Counts" , "Contributes further" , "Appends or increases" .)