Most restless but not starting to be most catty (8)
I believe the answer is:
itchiest
'most restless' is the definition.
('itchy' can be a synonym of 'restless')
'not starting to be most catty' is the wordplay.
'not starting' means to remove the first letter.
'to be most catty' becomes 'bitchiest' ('bitchy' can be a synonym of 'catty').
'bitchiest' with its first letter removed is 'ITCHIEST'.
'but' acts as a link.
(Other definitions for itchiest that I've seen before include "Most restless" , "extremely irritating" .)