Burglar lashes Tory leader! That's most malicious! (8)
I believe the answer is:
'most malicious' is the definition.
('catty' can be similar in meaning to 'malicious')
'burglar lashes tory leader' is the wordplay.
'burglar' becomes 'cat' (cat burglar).
'lashes' becomes 'ties'.
'tory leader' becomes 't' (I've seen this in other clues).
'that's' is the link.
(Another definition for cattiest that I've seen is " Most spiteful".)