Have to block that inept reformed Communist Party (5,3,4,3)
I believe the answer is:
paint the town red
'party' is the definition.
(to paint the town red is to party hard)
'have to block that inept reformed communist' is the wordplay.
'have' becomes 'own' (to have something is to own it).
'to block' means one lot of letters goes inside another (block can mean to plug a gap).
'reformed' indicates anagramming the letters.
'communist' becomes 'red' (red can informally mean communist).
'thatinept' is an anagram of 'paintthet'.
'own' put into 'paintthetred' is 'PAINT THE TOWN RED'.
(Other definitions for paint the town red that I've seen before include "enjoy celebration" , "get rid of the blues?" , "Celebrate" , "Go on a pub crawl" .)