Left-wingers on Brexit revolting in one go (4)
I believe the answer is:
brio
'go' is the definition.
(both can mean energy or enthusiasm)
'left-wingers on brexit revolting in one' is the wordplay.
'left-wingers on' says to take the initial letters (the letters on the left of the word).
The initial letters of 'brexit revolting in one' is 'BRIO'.
(Other definitions for brio that I've seen before include "Enthusiasm" , "In music, con ..... means with dash and verve" , "Vigour, vivacity" , "Spiritedness" , "(Of music) spirit" .)