Type of polecat is a diligent searcher (6)
I believe the answer is:
ferret
I believe this clue is a double definition.
'type of polecat' is the first definition.
'diligent searcher' is the second definition.
(I've seen this before)
'is a' is the link.
(Other definitions for ferret that I've seen before include "Small fierce animal - good hunting rabbits" , "It digs out rabbits" , "Small weasel good at hunting rabbits" , "Tame polecat" , "Animal to hunt rats and rabbits" .)