Socialist party upset stranger (5)
I believe the answer is:
odder
'stranger' is the definition.
(I've seen this before)
'socialist party upset' is the wordplay.
'socialist' becomes 'red' (red is a kind of socialist).
'party' becomes 'do' (do is a kind of party).
'upset' says the letters should be written in reverse.
'red'+'do'='reddo'
'reddo' written backwards gives 'ODDER'.
(Other definitions for odder that I've seen before include "More unusual" , "More peculiar" , "more rum" , "More strange or less even" , "Stranger" .)