A victory won at too great a cost to the victor (7)
I believe the answer is:
pyrrhic
'a victory won at too great a cost to the victor' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pyrrhic that I've seen before include "A victory won at such great cost that it's not a triumph" , "A victory won at too great a cost to the victor" , "Such a victory as bad as defeat" , "This victory comes at too great a price" , "(Of victory) where winner's losses are as great as loser's" .)