Socialist admits new split (4)
I believe the answer is:
rend
'split' is the definition.
(thesaurus)
'socialist admits new' is the wordplay.
'socialist' becomes 'red' (red is a kind of socialist).
'admits' means one lot of letters goes inside another.
'new' becomes 'n'.
'red' going around 'n' is 'REND'.
(Other definitions for rend that I've seen before include "Tear savagely" , "Cleave" , "Rip" , "Tear, sunder" , "split" .)