Joke about unusual, empty prison camp (5)
I believe the answer is:
gulag
'prison camp' is the definition.
(gulag is a kind of prison camp)
'joke about unusual empty' is the wordplay.
'joke' becomes 'gag' ('gag' can be a synonym of 'joke'**).
'about' indicates putting letters inside.
'empty' means to remove the middle letters.
'unusual' with its centre removed is 'ul'.
'gag' going around 'ul' is 'GULAG'.
(Other definitions for gulag that I've seen before include "Labour camp in the Soviet Union" , "where state's enemies were held" , "Soviet political prison system" , "USSR system of labour camps" , "Place for political dissidents" .)