Served up aspic in a tasting sample — that's nasty! (7)
I believe the answer is:
satanic
'nasty' is the definition.
(relating to Satan)
'served up aspic in a tasting sample' is the wordplay.
'served up' shows that the letters should be reversed in order (in a down clue, letters go up).
'sample' indicates the answer is hidden within the clue (I've seen 'sampled' mean this (I've seen this in other clues)).
'cinatas' can be found hidden inside 'aspic in a tasting'.
'cinatas' written backwards gives 'SATANIC'.
'that's' acts as a link.
(Other definitions for satanic that I've seen before include "Devil-like" , "Diabolic" , "Nick's" , "like Mephisto" , "Demonic" .)