Dogs crowd round headless corpse (8)
I believe the answer is:
mastiffs
'dogs' is the definition.
(mastiff is a kind of dog)
'crowd round headless corpse' is the wordplay.
'crowd' becomes 'mass' (massing is a kind of crowding**).
'round' is an insertion indicator.
'headless' means to remove the first letter.
'corpse' becomes 'stiff' ('stiff' can be a synonym of 'corpse').
'stiff' with its first letter taken off is 'tiff'.
'mass' going around 'tiff' is 'MASTIFFS'.
(Other definitions for mastiffs that I've seen before include "man's best friends?" , "Powerful deep-chested dogs" .)