Escort hugging Bill in darkness (6)
I believe the answer is:
shadow
'darkness' is the definition.
(I know that darkness can be written as shadow)
'escort hugging bill' is the wordplay.
'escort' becomes 'show' (I've seen this in other clues).
'hugging' means one lot of letters goes inside another.
'bill' becomes 'ad' (advertisement stuck on wall).
'show' enclosing 'ad' is 'SHADOW'.
'in' is the link.
(Other definitions for shadow that I've seen before include "Area of darkness caused by obstructing light" , "Trace - dog" , "Dark area - follow secretly" , "Follow closely - dark image - opposition" , "Dark area from which light is blocked" .)