Not even possibly right (5)
I believe the answer is:
never
'not' is the definition.
(thesaurus)
'even possibly right' is the wordplay.
'possibly' indicates anagramming the letters.
'right' becomes 'r' (abbreviation).
'even' with letters rearranged gives 'neve'.
'neve'+'r'='NEVER'
(Other definitions for never that I've seen before include "' - - Say - - Again': Bond film" , "Strong negative" , "At no time whatever" , "No way" , "Not in a million years!" .)