A leader of Brexiteers needs striking roughly (5)
I believe the answer is:
about
'roughly' is the definition.
(about can mean roughly or approximately)
'a leader of brexiteers needs striking' is the wordplay.
'leader of' says to take the initial letters.
'needs' says to put letters next to each other.
'striking' becomes 'out' (striking out is a common phrase).
The initial letter of 'brexiteers' is 'b'.
'a'+'b'+'out'='ABOUT'
(Other definitions for about that I've seen before include "Approximately, or in vicinity" , "Concerning (something)" , "Circa" , "In conection with, all around" , "in action" .)