A dog is a b..... good thing to have! (6)
I believe the answer is:
basset
'a dog' is the definition.
(type of dog)
'b good thing to have' is the wordplay.
'good thing to have' becomes 'asset' (I've seen this in another clue).
'b'+'asset'='BASSET'
'is a' acts as a link.
(Other definitions for basset that I've seen before include "Hare-hunting hound among beasts" , "Type of hound" , "Low, smooth-haired breed of hound" , "Beasts (anag.)" , "Low-set hound" .)