First defeat (4)
I believe the answer is:
best
This clue is a double definition.
'first' is the first definition.
(similar in meaning)
'defeat' is the second definition.
(I know that best is a type of vanquish)
(Other definitions for best that I've seen before include "Overcome" , "most fitting" , "Cream of the crop" , "George -, footballer" , "Most excellent; outwit" .)