More ghastly ingrained dirt by river across motorway (7)
I believe the answer is:
grimmer
'more ghastly' is the definition.
(grim can mean ghastly or bleak)
'ingrained dirt by river across motorway' is the wordplay.
'ingrained dirt' becomes 'grime' (I've seen this before).
'by' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
'across' means one lot of letters goes inside another.
'motorway' becomes 'M' (as in motorway names - Mg, Mm etc).
'grime'+'r'='grimer'
'grimer' enclosing 'm' is 'GRIMMER'.
(Another definition for grimmer that I've seen is " Darker".)