Terrible gala, nice cake decoration (8)
I believe the answer is:
angelica
'cake decoration' is the definition.
(angelica stalks are used to decorate cakes)
'terrible gala nice' is the wordplay.
'terrible' is an anagram indicator.
'gala'+'nice'='galanice'
'galanice' is an anagram of 'ANGELICA'.
(Other definitions for angelica that I've seen before include "Wild plant, often crystallised" , "may be sitting on top of the trifle" , "Herb" , "Candied stems of a plant" , "Nag Alice to get fragrant plant" .)