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' indicates an anagram.
'about' is an insertion indicator.
'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" .)