A b.... good thing you have a dog! (6)
I believe the answer is:
'a dog' is the definition.
(type of dog)
'a b good thing' is the wordplay.
'good thing' becomes 'asset' (an advantageous or desirable thing).
'b'+'asset'='BASSET'
'you have' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for basset that I've seen before include "Beasts (anag.)" , "Type of hound" , "Type of horn" , "Low, smooth-haired breed of hound" , "Sturdy hunting dog" .)