Communist Party backing stranger (5)
I believe the answer is:
odder
'stranger' is the definition.
(more odd)
'communist party backing' is the wordplay.
'communist' becomes 'red' (red can informally mean communist).
'party' becomes 'do' (do can informally mean a party).
'backing' says the letters should be written in reverse.
'red'+'do'='reddo'
'reddo' reversed gives 'ODDER'.
(Other definitions for odder that I've seen before include "more rum" , "More peculiar" , "More strange or less even" , "More unusual" , "Stranger" .)