As an alternative, detains criminal (7)
I believe the answer is:
instead
'as an alternative' is the definition.
('instead' can be a synonym of 'as an alternative')
'detains criminal' is the wordplay.
'criminal' indicates an anagram.
'detains' anagrammed gives 'INSTEAD'.
(Other definitions for instead that I've seen before include "for a change" , "Stained (anag)" , "In place (of)" , "Rather - detains (anag)" , "In lieu" .)