Inmates of Belsen freed otherwise (4)
I believe the answer is:
else
'otherwise' is the definition.
(synonyms)
'inmates of belsen freed' is the wordplay.
'inmates of' indicates the answer is hidden within the clue (letters imprisoned in the clue).
'ELSE' is hidden in the letters of 'belsen freed'.
(Other definitions for else that I've seen before include "In addition, besides" , "In addition; different" , "Adverb meaning if not, otherwise" , "As an alternative" , "Besides; instead" .)