Joke about regularly full Russian prison camp (5)
I believe the answer is:
gulag
'russian prison camp' is the definition.
(I've seen this before)
'joke about regularly full' is the wordplay.
'joke' becomes 'gag' ('gag' can be a synonym of 'joke'**).
'about' means one lot of letters goes inside another.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'full' are 'ul'.
'gag' placed around 'ul' is 'GULAG'.
(Other definitions for gulag that I've seen before include "Labour administration" , "place of detention" , "Russian prison and labour camp organisation" , "Russian labour camp system" , "Harsh labour camp" .)