Writing about mob in American volume (7)
I believe the answer is:
omnibus
'volume' is the definition.
(I have seen 'Compendium volume ' mean 'omnibus' so perhaps 'volume' could also mean 'omnibus')
'writing about mob in american' is the wordplay.
'writing about' is an anagram indicator (letters moved around or about).
'american' becomes 'US' (abbreviation for United States).
'mob'+'in'='mobin'
'mobin' anagrammed gives 'omnib'.
'omnib'+'us'='OMNIBUS'
(Other definitions for omnibus that I've seen before include "book collection" , "Large volume; means of transport" , "Comprehensive collection of books" , "Collection of writing" , "traffic" .)