Spiteful about right-winger, endlessly winning (10)
I believe the answer is:
victorious
'winning' is the definition.
(I know that winning can be written as victorious)
'spiteful about right-winger endlessly' is the wordplay.
'spiteful' becomes 'vicious' (I've seen this before).
'about' means one lot of letters goes inside another.
'right-winger' becomes 'Tory' (Tory is an example).
'endlessly' means to remove the last letter.
'tory' with its last letter removed is 'tor'.
'vicious' enclosing 'tor' is 'VICTORIOUS'.
(Other definitions for victorious that I've seen before include "Triumphant" , "winning" .)