Possibly avenge it, possibly not (8)
I believe the answer is:
negative
'not' is the definition.
(thesaurus)
'possibly avenge it' is the wordplay.
'possibly' indicates anagramming the letters.
'avenge'+'it'='avengeit'
'avengeit' is an anagram of 'NEGATIVE'.
'possibly' acts as a link.
(Other definitions for negative that I've seen before include "Defeatist" , "A test result" , "Less than zero quantity" , "Expressing refusal, denial or disagreement" , "thumbs down" .)