Clue: Cunning Brexiteer speaking in support of Chequers at first (6)
Answer: CLEVER
Explanation: Definition: Cunning
Homophone of **LEAVER** ("Brexiteer" speaking) *in support of* **C**(hequers) [at first] 
Hate this clue, as the LEVER part of CLEVER is not pronounces as LEAVER.