Defence of jails in brig occasionally (5)
I believe the answer is:
alibi
'defence' is the definition.
(defence given to police by a suspect)
'jails in brig occasionally' is the wordplay.
'occasionally' indicates alternate letters.
'jails'+'in'+'brig'='jailsinbrig'
The alternating letters of 'jailsinbrig' are 'ALIBI'.
'of' acts as a link.
(Other definitions for alibi that I've seen before include "Making excuse" , "Reason often given to exculpate someone" , "Evidence of (or excuse for) being somewhere else at the time" , "Excuse, in law" , "Proof of not being at the scene of the crime" .)