Nothing possibly said about a parting word (5)
I believe the answer is:
adios
'a parting word' is the definition.
('goodbye' in Spanish)
'nothing possibly said about' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'possibly' is an anagram indicator.
'about' means one lot of letters goes inside another.
'said' anagrammed gives 'adis'.
'o' inserted within 'adis' is 'ADIOS'.
(Other definitions for adios that I've seen before include "Goodbye (Spanish)" , "Goodbye (20 across)" , "Goodbye to Spain" , "See you in Ibiza" , "Goodbye, senor" .)