Untruth maintained by Capone's associates (6)
I believe the answer is:
allies
'associates' is the definition.
(ally is a kind of associate)
'untruth maintained by capone's' is the wordplay.
'untruth' becomes 'lie' (lie is a kind of untruth).
'maintained by' is an insertion indicator (maintain can mean to keep hold of).
'capone's' becomes 'als'.
'lie' going inside 'als' is 'ALLIES'.
(Other definitions for allies that I've seen before include "Countries working together" , "Countries that opposed Hitler" , "They join and fight together" , "Political friends" , "Fellow-members, confederates" .)