Yankee invading force endlessly seeks victims (5)
I believe the answer is:
preys
'seeks victims' is the definition.
(as in preying on victims)
'yankee invading force endlessly' is the wordplay.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'invading' indicates putting letters inside.
'force endlessly' becomes 'pres' (I can't justify this - if you can you should believe this answer much more).
'y' going inside 'pres' is 'PREYS'.
(Other definitions for preys that I've seen before include "Predates" , "quarries" , "Weighs heavily (upon the mind)" , "-- upon; hunts" .)