Malicious or terribly up itself (8)
I believe the answer is:
spiteful
'malicious' is the definition.
('spiteful' can be similar in meaning to 'malicious')
'terribly up itself' is the wordplay.
'terribly' is an anagram indicator.
'up'+'itself'='upitself'
'upitself' anagrammed gives 'SPITEFUL'.
'or' acts as a link.
(Other definitions for spiteful that I've seen before include "nasty!" , "Vengeful, malicious" , "Showing malice" , "Catty" , "Vindictive - malicious" .)