Accused’s defence fails if bail oddly withheld (5)
I believe the answer is:
alibi
'accused's defence' is the definition.
(defence used when accused of a crime)
'fails if bail oddly withheld' is the wordplay.
'oddly withheld' indicates alternate letters (letters in odd-numbered positions removed).
'fails'+'if'+'bail'='failsifbail'
The alternate letters of 'failsifbail' are 'ALIBI'.
(Other definitions for alibi that I've seen before include "Claim put to court" , "Accused person's defence" , "Proof of being somewhere else at the time" , "Evidence one was not present" , "Evidence of (or excuse for) being somewhere else at the time" .)