Is among those left to fight (6)
I believe the answer is:
resist
'to fight' is the definition.
(I know that resist is a more specific form of defend)
'is among those left' is the wordplay.
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'those left' becomes 'rest' (I've seen this in other clues).
'is' going within 'rest' is 'RESIST'.
(Other definitions for resist that I've seen before include "Offer opposition to" , "Put up opposition, defy" , "Don't give in" , "Take stand against, oppose" , "Withstand or defy" .)