Mean not to fall in the river! (6)
I believe the answer is:
denote
'mean' is the definition.
(I know that denote is a more specific form of the action mean)
'not to fall in the river' is the wordplay.
'to fall in' means one lot of letters goes inside another.
'the river' becomes 'dee' (River Dee).
'not' inserted inside 'dee' is 'DENOTE'.
(Other definitions for denote that I've seen before include "Point" , "Designate" , "Indicate, signify" , "Be an indication of" , "Indicate, be a sign of" .)