Conservative is livid (4)
I believe the answer is:
blue
This clue is a double definition.
'conservative' is the first definition.
(I've seen this before)
'livid' is the second definition.
(thesaurus)
'is' acts as a link.
(Other definitions for blue that I've seen before include "Spend all at once - colour" , "Dispirited in cerulean way" , "eg Prussian" , "adult" , "Sad; butterfly" .)