Managed to collar canine that's released bit of ghastly gas (5)
I believe the answer is:
radon
'gas' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
(Other definitions for radon that I've seen before include "Radioactive gas, linked to granite" , "Element, symbol Rn" , "A radioactive gas, symbol Rn" , "Inert gas - adorn (anag)" , "Rare noble gas" .)