Brexit starts with Conservatives getting optimistic (6)
I believe the answer is:
bright
'optimistic' is the definition.
(thesaurus)
'brexit starts with conservatives getting' is the wordplay.
'starts with' says to take the initial letters (I've seen 'start with' mean this).
'conservatives' becomes 'right' (I've seen this in other clues).
'getting' says to put letters next to each other.
The first letter of 'brexit' is 'b'.
'b'+'right'='BRIGHT'
(Other definitions for bright that I've seen before include "full of promise" , "Shiny" , "Sparkling" , "7 [CLEVER]" , "Effulgent" .)