Even the finest can be overrun! (6)
I believe the answer is:
infest
'overrun' is the definition.
('infest' can be a synonym of 'overrun')
'even the finest can' is the wordplay.
'finest' can be anagrammed to 'INFEST'.
However, I'm not sure how the anagram is indicated.
'be' acts as a link.
Can you help me to learn more?
(Other definitions for infest that I've seen before include "Hostile once" , "Feints to be numerous and troublesome" , "Annoyingly swarm through" , "(E.g. vermin) abound in" , "Finest (anag)" .)