Common sort joining socialist (7)
I believe the answer is:
kindred
'common' is the definition.
(thesaurus)
'sort joining socialist' is the wordplay.
'sort' becomes 'kind' ('kind' can be a synonym of 'sort').
'joining' says to put letters next to each other.
'socialist' becomes 'red' (red is a kind of socialist).
'kind'+'red'='KINDRED'
(Other definitions for kindred that I've seen before include "Relatively" , "Related; similar in kind" , "Affinity" , "[TRIBAL]" , "These spirits have similar tastes" .)