Republican arrested in bar? That’s only part of the story (7)
I believe the answer is:
excerpt
'part of the story' is the definition.
(excerpt from a book)
'republican arrested in bar?' is the wordplay.
'republican' becomes 'R' (abbreviation).
'arrested' is an insertion indicator (arrest can mean to hold or imprison).
'in bar?' becomes 'except' (I've seen this before).
'r' going within 'except' is 'EXCERPT'.
'that's only' acts as a link.
(Other definitions for excerpt that I've seen before include "Passage from book, play etc" , "bit of work" , "Literary section" , "Passage selected from a book" , "Passage from literature or music" .)