Greene novel about opposing retaliation (7)
I believe the answer is:
revenge
'retaliation' is the definition.
('revenge' can be a synonym of 'retaliation')
'greene novel about opposing' is the wordplay.
'novel' is an anagram indicator.
'about' indicates putting letters inside.
'opposing' becomes 'v' (abbreviation for versus).
'greene' is an anagram of 'reenge'.
'reenge' going around 'v' is 'REVENGE'.
(Other definitions for revenge that I've seen before include "Malicious retaliation" , "Satisfaction taken for injury" , "Retribution" , "An eye for an eye" , "Ship remembered in verse" .)