Left-leaning parties saving time where men were detained (6)
I believe the answer is:
stalag
'where men were detained' is the definition.
(German prison camp in World War 2)
'left-leaning parties saving time' is the wordplay.
'left-leaning' says the letters should be written backwards (letters written right to left).
'parties' becomes 'galas' (a gala is a party or social event).
'saving' is an insertion indicator.
'time' becomes 't' (abbreviation).
'galas' going around 't' is 'galats'.
'galats' back-to-front is 'STALAG'.
(Other definitions for stalag that I've seen before include "Type of German prison camp" , "German prison camp in WW2" , "German concentration camp" , "German prison_camp" , "WWII German prisoner of war camp" .)