Book half of rabble, as lawman (7)
I believe the answer is:
sheriff
'lawman' is the definition.
(sheriff is a kind of lawman)
'book half of rabble' is the wordplay.
'book' becomes 'she' (I've seen this before**).
'half of' suggests halving the letters.
'rabble' becomes 'riffraff' ('riffraff' can be a synonym of 'rabble').
'riffraff' cut in half is 'riff'.
'she'+'riff'='SHERIFF'
'as' acts as a link.
(Other definitions for sheriff that I've seen before include "US law enforcement officer" , "US county law enforcer" , "Head of the posse" , "Lawman of the wild west" , "Chief Crown representative in a county" .)