Dreadful, and almost entirely accepted by Liverpudlian, mostly (10)
I believe the answer is:
scandalous
'dreadful' is the definition.
(thesaurus)
'and almost entirely accepted by liverpudlian mostly' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'entirely' becomes 'all' (all can mean entirely or completely).
'accepted by' is an insertion indicator (inserted letters are accepted).
'liverpudlian' becomes 'scouse' (both mean coming from Liverpool).
'mostly' means to remove the last letter.
'all' with its last letter taken off is 'al'.
'and'+'al'='andal'
'scouse' with its final letter removed is 'scous'.
'andal' inserted into 'scous' is 'SCANDALOUS'.
(Other definitions for scandalous that I've seen before include "Causing outrage" , "Shocking and offensive" , "Outrageous" , "Highly improper" , "Disgraceful" .)