Mark is not in river (6)
I believe the answer is:
denote
'mark' is the definition.
(I've seen this in another clue)
'not in river' is the wordplay.
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'not' inserted inside 'dee' is 'DENOTE'.
'is' is the link.
(Other definitions for denote that I've seen before include "show" , "Indicate, be a sign of" , "Point" , "Indicate, signify" , "Designate" .)