Prison uniform left in block (5)
I believe the answer is:
gulag
'prison' is the definition.
(gulag is a kind of prison)
'uniform left in block' is the wordplay.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'block' becomes 'gag' (I can't justify this - if you can you should believe this answer much more).
'u'+'l'='ul'
'ul' going into 'gag' is 'GULAG'.
Can you help me to learn more?
(Other definitions for gulag that I've seen before include "place of detention" , "Russian prison and labour camp organisation" , "Labour camp in the Soviet Union" , "System of detention camps in former Soviet Union" , "Labour administration" .)