Bill's in river, a very low stretch of water (4,3)
I believe the answer is:
dead sea
'very low stretch of water' is the definition.
(the Dead Sea is below sea level)
'bill's in river a' is the wordplay.
'bill's' becomes 'ads' (advertisement stuck on wall).
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'ads' put inside 'dee' is 'deadse'.
'deadse'+'a'='DEAD SEA'
(Other definitions for dead sea that I've seen before include "'Body of water, sounds like it's passed on (4,3)'" , "The lake that's a late ocean" , "Hypersaline lake" , "Saltwater lake in the Jordan valley" , "R. Jordan lake" .)