Nick is about to back proportional representation (6)
I believe the answer is:
prison
'nick' is the definition.
(British slang)
'is about to back proportional representation' is the wordplay.
'about' becomes 'on' (as in a book 'on' a certain topic).
'to back' means one lot of letters go next to another (some letters go at the back of others).
'proportional representation' becomes 'PR' (abbreviation).
'is'+'on'='ison'
'ison' after 'pr' is 'PRISON'.
(Other definitions for prison that I've seen before include "Wormwood Scrubs, for example" , "cage" , "Penitentiary" , "Cooler" , "Slammer" .)