Least shallow river has vermin (7)
I believe the answer is:
deepest
'least shallow' is the definition.
('deep' is the opposite of 'shallow' so 'deepest' should be a synonym for 'least shallow')
'river has vermin' is the wordplay.
'river' becomes 'dee' (River Dee).
'has' means one lot of letters go next to another.
'vermin' becomes 'pest'.
'dee'+'pest'='DEEPEST'
(Other definitions for deepest that I've seen before include "most hard to fathom" , "most insidious" , "Of the greatest depth" , "most demanding" , "Most low" .)