...prosper containing socialist apathy (7)
I believe the answer is:
boredom
'apathy' is the definition.
(thesaurus)
'prosper containing socialist' is the wordplay.
'prosper' becomes 'boom' ('boom' can be a synonym of 'prosper').
'containing' means one lot of letters goes inside another.
'socialist' becomes 'red' (red is a kind of socialist).
'boom' enclosing 'red' is 'BOREDOM'.
(Other definitions for boredom that I've seen before include "Tedium, ennui" , "Ennui in bedroom" , "weary idleness" , "Mental state" , "Monotony" .)