Author possibly sad about a sum being reduced (5)
I believe the answer is:
dumas
'author' is the definition.
(I've seen this before)
'sad about a sum being reduced' is the wordplay.
'about' indicates an anagram.
'being reduced' means to remove the first letter.
'sum' with its first letter taken away is 'um'.
'sad' anagrammed with 'um' makes 'DUMAS'.
'possibly' acts as a link.
(Other definitions for dumas that I've seen before include "Author of The Three Musketeers" , "French novelists, father and son" , "Musketeers writer" , "French pair who wrote" , "Alexandre ---, wrote Count of Monte Cristo" .)