A socialist gets to sit back inside, being detained (8)
I believe the answer is:
arrested
'being detained' is the definition.
(thesaurus)
'a socialist gets to sit back' is the wordplay.
'socialist' becomes 'red' (socialism is often associated with the colour red).
'gets' is an insertion indicator (gets can mean captures or absorbs).
'to sit back' becomes 'rest' (as in sitting back and relaxing).
'red' going around 'rest' is 'rrested'.
'a'+'rrested'='ARRESTED'
'inside' acts as a link.
(Other definitions for arrested that I've seen before include "Seized and took into custody" , "Stopped or slowed down development" , "Taken into police custody" , "Detained by legal authority" , "Checked" .)