Mean not to enter river (6)
I believe the answer is:
denote
'mean' is the definition.
(denoting is a kind of meaning)
'not to enter river' is the wordplay.
'to enter' means one lot of letters goes inside another.
'river' becomes 'dee' (River Dee).
'not' going inside 'dee' is 'DENOTE'.
(Other definitions for denote that I've seen before include "Indicate, signify" , "Be an indication of" , "Designate" , "Indicate, be a sign of" , "Be the sign or symbol of" .)