Grimly cynical (8)
I believe the answer is:
sardonic
'grimly cynical' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sardonic that I've seen before include "Sarcastic" , "Dry, cynically mocking" , "Derisively ironic" , "Cynical -- so rancid (anag)" , "Humorous in grim way - seems so rancid" .)