Overrun with finest nuts (6)
I believe the answer is:
infest
'overrun' is the definition.
(synonyms)
'finest nuts' is the wordplay.
'nuts' is an anagram indicator (nuts can mean crazy).
'finest' is an anagram of 'INFEST'.
'with' acts as a link.
(Other definitions for infest that I've seen before include "Overrun" , "Unwelcomely occupy" , "plague" , "(Of insects) to fill or invade (something)" , "Numerically swamp" .)