If not, I swore to release the captive (9)
I believe the answer is:
otherwise
'if not' is the definition.
(synonyms)
'i swore to release the captive' is the wordplay.
'to release' indicates an anagram (I've seen 'released' mean this (letters released from their original order)).
'captive' means one lot of letters goes inside another.
'i'+'swore'='iswore'
'iswore' with letters rearranged gives 'orwise'.
'orwise' enclosing 'the' is 'OTHERWISE'.
(Other definitions for otherwise that I've seen before include "Alternatively" , "Apart from that" , "White rose seen in a different way" , "White rose (anag.)" , "Under different conditions" .)