Defence from gangster, one getting bail occasionally (5)
I believe the answer is:
alibi
'defence' is the definition.
(defence given to police by a suspect)
'gangster one getting bail occasionally' is the wordplay.
'gangster' becomes 'al' (Al Capone).
'one' becomes 'i' (Roman numeral).
'getting' means one lot of letters go next to another.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
The alternate letters of 'bail' are 'bi'.
'al'+'i'+'bi'='ALIBI'
'from' acts as a link.
(Other definitions for alibi that I've seen before include "It shows one wasn't at crime scene" , "Evidence one was not present" , "Defence against accusation - was seen elsewhere at the time" , "Excuse, in law" , "Legal excuse" .)