Terrible vice: need support (8)
I believe the answer is:
evidence
'support' is the definition.
(I've seen this before)
'terrible vice need' is the wordplay.
'terrible' is an anagram indicator.
'vice'+'need'='viceneed'
'viceneed' anagrammed gives 'EVIDENCE'.
(Other definitions for evidence that I've seen before include "Testimony or outward sign" , "Pressing necessity" , "Info given in court" , "Witness's testimony" , "Facts and figures relevant to a case" .)